Nickel-titanium alloy including a rare earth element

ABSTRACT

Disclosed herein is a nickel-titanium alloy comprising nickel, titanium, and at least one rare earth element. The nickel-titanium alloy comprises from about 34 at. % to about 60 at. % nickel, from about 34 at. % to about 60 at. % titanium, and from about 0.1 at. % to about 15 at. % at least one rare earth element. The nickel-titanium alloy may further include one or more additional alloying elements. In addition to radiopacity, the nickel-titanium alloy preferably exhibits superelastic or shape memory behavior. Medical devices comprising the nickel-titanium alloy and a method of making them are also disclosed.

RELATED APPLICATIONS

The present patent document claims the benefit of the filing date under35 U.S.C. §119(e) of Provisional U.S. Patent Application Ser. No.60/842,484, filed Sep. 6, 2006, Provisional U.S. Patent Application Ser.No. 60/872,369, filed Nov. 29, 2006, and Provisional U.S. PatentApplication Ser. No. 60/872,370, filed Nov. 29, 2006, which are herebyincorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to nickel-titanium alloys andmore particularly, but not exclusively, to nickel-titanium alloys formedical device applications.

BACKGROUND

Nickel-titanium alloys are commonly used for the manufacture ofintraluminal biomedical devices, such as self-expandable stents, stentgrafts, embolic protection filters, and stone extraction baskets. Suchdevices may exploit the superelastic or shape memory behavior ofequiatomic or near-equiatomic nickel-titanium alloys, which are commonlyreferred to as Nitinol.

As a result of the poor radiopacity of nickel-titanium alloys, however,such devices may be difficult to visualize from outside the body usingnon-invasive imaging techniques, such as x-ray fluoroscopy.Visualization is particularly problematic when the intraluminal deviceis made of fine wires or thin-walled struts. Consequently, a clinicianmay not be able to accurately place and/or manipulate a Nitinol stent orbasket within a body vessel.

Current approaches to improving the radiopacity of nickel-titaniummedical devices include the use of radiopaque markers or coatings. Forexample, gold markers attached to ends of a stent may guide thepositioning of the device and delineate its length during an x-rayprocedure. Alternatively, a medical device may be plated, clad orotherwise coated with gold or another heavy metal to create a radiopaquesurface or outer layer. In another approach, a heavy metal cylinder maybe included within the lumen of a stent to produce a radiopaque core.These approaches to improving radiopacity may have shortcomings,however. In some cases, markers may be easily dislodged or mayundesirably increase the delivery profile of the device. A surfacecoating or cladding may delaminate as the medical device is expanded orit may interfere with the mechanical behavior of the device. Radiopaquecores may be expensive to fabricate. Galvanic corrosion may also be aproblem. Furthermore, gold and other heavy metals, such as platinum,palladium, and tungsten, tend to be costly.

BRIEF SUMMARY

Disclosed herein is a nickel-titanium alloy comprising nickel, titanium,and at least one rare earth element. The nickel-titanium alloy mayfurther include one or more additional alloying elements. In addition toradiopacity, the nickel-titanium alloy preferably exhibits superelasticor shape memory behavior. Medical devices comprising the nickel-titaniumalloy and a method of making and using them are also disclosed.

According to one embodiment, the nickel-titanium alloy comprises fromabout 34 at. % (atomic percent) to about 60 at. % nickel, from about 34at. % to about 60 at. % titanium, and from about 0.1 at. % to about 15at. % at least one rare earth element selected from the group consistingof La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa,and U.

According to another embodiment, the nickel-titanium alloy comprisesfrom about 34 at. % to about 60 at. % nickel, from about 34 at. % toabout 60 at. % titanium, and from about 0.1 at. % to about 15 at. % atleast one rare earth element, whereby the nickel-titanium alloy has aradiopacity greater than that of a near-equiatomic binarynickel-titanium alloy.

According to another embodiment, the nickel-titanium alloy includes fromabout 34 at. % to about 60 at. % nickel, from about 34 at. % to about 60at. % titanium, from about 0.1 at. % to about 10 at. % at least one rareearth element, and at least one transition metal at a concentration ofno more than about 4.9 at. %. The rare earth element is selected fromthe group consisting of Ce, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Ac, Th, Pa, and U.

According to another embodiment, the nickel-titanium alloy comprisesfrom about 34 at. % to about 60 at. % nickel, from about 34 at. % toabout 60 at. % titanium, from about 0.1 at. % to about 10 at. % at leastone rare earth element; and at least one transition metal at aconcentration of no more than about 4.9 at. %, wherein the at least onerare earth element is selected from the group consisting of Ce, La, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, and U, andwhereby the nickel-titanium alloy has a radiopacity greater than that ofa near-equiatomic binary nickel-titanium alloy.

According to one embodiment, the medical device comprises at least onecomponent including a nickel-titanium alloy including from about 34 at.% to about 60 at. % nickel, from about 34 at. % to about 60 at. %titanium, and from about 0.1 at. % to about 15 at. % at least one rareearth element, wherein the at least one rare earth element is selectedfrom the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Ac, Th, Pa and U.

According to another embodiment, the medical device comprises at leastone component comprising a nickel-titanium alloy including from about 34at. % to about 60 at. % nickel; from about 34 at. % to about 60 at. %titanium; from about 0.1 at. % to about 10 at. % at least one rare earthelement; and at least one transition metal at a concentration of no morethan about 4.9 at. %, wherein the at least one rare earth element isselected from the group consisting of Ce, La, Pr, Nd, Pm, Sm, Eu, Gd,Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, and U.

According to another embodiment, the medical device comprises least onecomponent comprising a nickel-titanium alloy including nickel at aconcentration of from about 34 at. % to about 60 at. %, titanium at aconcentration of from about 34 at. % to about 60 at. %, and at least onerare earth element at a concentration of from about 0.1 at. % to about15 at. %, wherein the at least one rare earth element is selected fromthe group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Yb, Lu, Ac, Th, Pa and U. The nickel-titanium alloy comprises aradiopacity greater than that of a near-equiatomic binarynickel-titanium alloy, and the nickel-titanium alloy further comprises arecoverable strain of at least about 0.5% upon removal of a deformingstress at or below body temperature.

Also disclosed herein is a method of using a medical device. To carryout the method, according to one aspect, a medical device including atleast one component comprising from about 34 at. % to about 60 at. %nickel, from about 34 at. % to about 60 at. % titanium, and from about0.1 at. % to about 15 at. % at least one rare earth element is provided.The rare earth element is selected from the group consisting of La, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, and U. Themedical device is then delivered to a treatment site within the patient.

According to another embodiment, the method of using the medical devicecomprises providing a medical device comprising at least one componentcomprising a nickel-titanium alloy including from about 34 at. % toabout 60 at. % nickel, from about 34 at. % to about 60 at. % titanium,from about 0.1 at. % to about 10 at. % at least one rare earth element,and at least one transition metal at a concentration of no more thanabout 4.9 at. %, wherein the at least one rare earth element is selectedfrom the group consisting of Ce, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, Lu, Ac, Th, Pa, and U, and delivering the medical device toa treatment site within the patient.

Also disclosed is a method of imaging a medical device within a patient.The method comprises, according to one aspect, delivering a medicaldevice having at least one component made from a nickel-titanium alloyincluding from about 34 at. % to about 60 at. % nickel, from about 34at. % to about 60 at. % titanium, and from about 0.1 at. % to about 15at. % at least one rare earth element to a site in a patient. The atleast one rare earth element is selected from the group consisting ofLa, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, andU. The patient is then exposed to radiation having an energy in therange of from 15 keV to 150 keV to image the medical device.

According to another aspect, the method of imaging the medical devicewithin a patient comprises delivering a medical device having at leastone component comprising a nickel-titanium alloy including from about 34at. % to about 60 at. % nickel, from about 34 at. % to about 60 at. %titanium, from about 0.1 at. % to about 10 at. % at least one rare earthelement, and at least one transition metal at a concentration of no morethan about 4.9 at. % to a site in a patient. The at least one rare earthelement is selected from the group consisting of Ce, La, Pr, Nd, Pm, Sm,Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, and U. The patient isthen exposed to radiation having an energy in the range of from 15 keVto 150 keV to image the medical device.

In addition, a method of making a medical device is disclosed. To carryout the method, a melt including from about 34 at. % to about 60 at. %nickel, from about 34 at. % to about 60 at. % titanium, and from about0.1 at. % to about 15 at. % a rare earth element is formed. The rareearth element is selected from the group consisting of La, Pr, Nd, Pm,Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, and U. The melt iscooled to form a solid, and the solid is formed into a component to forma medical device.

According to another aspect, the method of making the medical devicecomprises forming a melt comprising from about 34 at. % to about 60 at.% nickel, from about 34 at. % to about 60 at. % titanium, from about 0.1at. % to about 10 at. % at least one rare earth element, and at leastone transition metal at a concentration of no more than about 4.9 at. %,wherein the at least one rare earth element is selected from the groupconsisting of Ce, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,Lu, Ac, Th, Pa, and U. The melt is cooled to form a solid, and the solidis formed into a component to form the medical device.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the present invention will now be described, byway of example, with reference to the accompanying drawings, in which:

FIG. 1 is a ternary alloy concentration diagram showing a compositionrange of a nickel-titanium alloy according to one embodiment;

FIG. 2 is a graph of linear absorption coefficient versus photon energyfor several rare earth (RE) elements and platinum;

FIG. 3 is a graph of the linear absorption coefficient of FIG. 2normalized with respect to the linear absorption coefficient of platinumversus photon energy;

FIG. 4A is graph of relative incidence versus photon energy at a 40 kVptube voltage for four different filtration levels;

FIG. 4B is graph of relative incidence versus photon energy at a 70 kVptube voltage for four different filtration levels;

FIG. 4C is graph of relative incidence versus photon energy at a 80 kVptube voltage for four different filtration levels;

FIG. 4D is graph of relative incidence versus photon energy at a 125 kVptube voltage for four different filtration levels;

FIG. 5 is a graph of the calculated cumulative linear absorptioncoefficient of various Ni—Ti-RE alloys for a 40 kVp tube voltage andseveral filtration schemes;

FIG. 6 is a graph of the calculated cumulative linear absorptioncoefficient (radiopacity) of various Ni—Ti-RE alloys for a 70 kVp tubevoltage and several filtration schemes;

FIG. 7 is a graph of the calculated cumulative linear absorptioncoefficient (radiopacity) of various Ni—Ti-RE alloys for a 80 kVp tubevoltage and several filtration schemes;

FIG. 8 is a graph of the calculated cumulative linear absorptioncoefficient (radiopacity) of various Ni—Ti-RE alloys for a 125 kVp tubevoltage and several filtration schemes;

FIG. 9 is a graph of the radiopacity of various Ni—Ti-RE alloys relativeto the radiopacity of a near-equiatomic binary nickel-titanium alloy fora 40 kVp tube voltage and several filtration schemes;

FIG. 10 is a graph of the radiopacity of various Ni—Ti-RE alloysrelative to that of near-equiatomic binary nickel-titanium alloy for a70 kVp tube voltage and several filtration schemes;

FIG. 11A and FIG. 11B are graphs of the radiopacity of various Ni—Ti-REalloys relative to that of a near-equiatomic binary nickel-titaniumalloy for a 80 kVp tube voltage and several filtration schemes;

FIG. 12 is a graph of the radiopacity of various Ni—Ti-RE alloysrelative to that of near-equiatomic binary nickel-titanium alloy for a125 kVp tube voltage and several filtration schemes.

FIG. 13 is a diagram of stress versus strain for an exemplary shapememory alloy at a temperature above an austenitic final temperature ofthe alloy;

FIG. 14 is a transformation temperature curve for an exemplary shapememory alloy;

FIG. 15 is a diagram of strain versus temperature for an exemplary shapememory alloy;

FIG. 16 is a schematic of an exemplary wire having a composite structureincluding at least one layer formed of a Ni—Ti-RE alloy;

FIG. 17A is a cross-sectional schematic of an exemplary cable formedfrom seven wire strands, where one or more of the strands are formed ofa Ni—Ti-RE alloy;

FIG. 17B is a side view schematic of an exemplary braided wirestructure, where one or more strands of the wire structure are formed ofa Ni—Ti-RE alloy;

FIG. 18 is a schematic of an exemplary cannula having a compositestructure, where one or more layers of the cannula are formed of aNi—Ti-RE alloy;

FIG. 19 is a schematic of an exemplary stent formed from one or morewires, where all or a portion of the wires is formed of a Ni—Ti-REalloy;

FIG. 20 is a schematic of a phantom developed by the Center for Devicesand Radiological Health (CDRH) to simulate x-ray attenuation through thelower abdomen of a typical adult;

FIG. 21 is a bar graph showing the average improvement in x-ray contrastfor Ni—Ti—X specimens (X═Gd, Er or Pt) relative to binary Ni—Ti asdetermined using the CDRH phantom at various tube voltages influoroscopic mode; and

FIG. 22 is a bar graph showing the average improvement in x-ray contrastfor Ni—Ti—X specimens (X═Gd, Er or Pt) relative to binary Ni—Ti asdetermined using the CDRH phantom at various tube voltages in staticmode.

DETAILED DESCRIPTION Definitions

As used in the following specification and the appended claims, thefollowing terms will have the meanings ascribed below:

Martensite start temperature (M_(s)) is the temperature at which a phasetransformation to martensite begins upon cooling for a shape memorymaterial exhibiting a martensitic phase transformation.

Martensite finish temperature (M_(f)) is the temperature at which thephase transformation to martensite concludes upon cooling.

Austenite start temperature (A_(s)) is the temperature at which a phasetransformation to austenite begins upon heating for a shape memorymaterial exhibiting an austenitic phase transformation.

Austenite finish temperature (A_(f)) is the temperature at which thephase transformation to austenite concludes upon heating.

R′-phase start temperature (R′_(s)) is the temperature at which a phasetransformation to R-phase begins upon heating for shape memory materialexhibiting an R-phase transformation.

R′-phase finish temperature (R′_(f)) is the temperature at which thephase transformation to R-phase concludes upon heating.

R-phase start temperature (R_(s)) is the temperature at which a phasetransformation to R-phase begins upon cooling for a shape memorymaterial exhibiting an R-phase transformation.

R-phase finish temperature (R_(f)) is the temperature at which the phasetransformation to R-phase concludes upon cooling.

Radiopacity is a measure of the capacity of a material or object toabsorb incident electromagnetic radiation, such as x-ray radiation. Aradiopaque material preferentially absorbs incident x-rays and tends toshow high radiation contrast and good visibility in x-ray images. Amaterial that is not radiopaque tends to transmit incident x-rays andmay not be readily visible in x-ray images. A linear absorptioncoefficient (μ) of a material may be a good indicator of its capacityfor absorbing x-ray radiation, and thus its radiopacity. For thepurposes of this disclosure, a cumulative linear absorption coefficient,which is defined and described in detail below, may be taken asrepresentative of the radiopacity of a material.

The term “near-equiatomic binary nickel-titanium alloy” refers to atwo-component alloy including from 45 at. % to 55 at. % nickel and thebalance titanium.

Described herein is a nickel-titanium alloy comprising nickel, titanium,and at least one rare earth element. According to one embodiment, thenickel-titanium alloy comprises at least one additional alloyingelement. The nickel-titanium alloy preferably has improved radiopacitycompared to previous nickel-titanium alloys. Accordingly, a medicaldevice comprising the nickel-titanium alloy may have better visibilityduring non-invasive imaging procedures such as x-ray fluoroscopy. Thenickel-titanium alloy preferably has superelastic or shape memoryproperties that are advantageous for medical devices, as will bediscussed below.

Preferably, the one or more rare earth elements of the nickel-titaniumalloy are chosen from the lanthanide series and/or the actinide seriesof the periodic table, which include La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, and U. Yttrium (Y) and scandium (Sc)are sometimes referred to as rare earth elements although they are notelements of the lanthanide or actinide series. More preferably, the rareearth (RE) element is selected from the group consisting of La, Pr, Nd,Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

According to a preferred embodiment, the rare earth element occupies aternary position in terms of concentration in the alloy. In other words,the amount of the rare earth element is preferably less than therespective amounts of nickel and titanium, but greater than the amountof any additional alloying elements that may be present in the alloy. Anexemplary composition range for the alloy is shown schematically in FIG.1.

The nickel-titanium alloy comprises at least about 0.1% at least onerare earth element, according to one embodiment. Preferably, thenickel-titanium alloy comprises at least about 1.0 at. % at least onerare earth element. More preferably, the nickel-titanium alloy comprisesat least about 2.5 at. % at least one rare earth element. It may bedesirable that the nickel-titanium alloy comprises at least about 5 at.% at least one rare earth element.

It is also preferred that the nickel-titanium alloy comprises no morethan about 15 at. % at least one rare earth element. More preferably,the nickel-titanium alloy comprises no more than about 12.5 at. % atleast one rare earth element. Even more preferably, the nickel-titaniumalloy comprises no more than about 10 at. % at least one rare earthelement. Yet even more preferably, the nickel-titanium alloy comprisesno more than about 7.5 at. % at least one rare earth element. Mostpreferably, the nickel-titanium alloy comprises no more than about 5.0at. % at least one rare earth element.

By way of example, the nickel-titanium alloy comprises from about 0.1at. % to about 15 at. % at least one rare earth element, according to apreferred embodiment. Preferably, the nickel-titanium alloy comprisesfrom about 1.0 at. % to about 12.5 at. %. More preferably, thenickel-titanium alloy comprises from about 1.0 at. % to about 10.0 at. %at least one rare earth element. Even more preferably, thenickel-titanium alloy comprises from about 1.0 at. % to about 7.5 at. %,or from about 2.5 at. % to about 7.5 at. % at least one rare earthelement. Most preferably, the nickel-titanium alloy comprises from about2.5 at. % to about 5.0 at. % at least one rare earth element. Forexample, the nickel-titanium alloy may comprise 3.0 at. %, 3.25 at. %,3.5 at. %, 3.75 at. % or 4 at. % at least one rare earth element.

According to a preferred embodiment, the nickel-titanium alloy comprisesat least about 34 at. % nickel. More preferably, the nickel-titaniumalloy comprises at least about 36.5 at. % nickel. Even more preferably,the nickel-titanium alloy comprises at least about 39 at. % nickel.Still more preferably, the nickel-titanium alloy comprises at leastabout 44 at. % nickel.

It is also preferred that the nickel-titanium alloy comprises no morethan about 60 at. % nickel. More preferably, the nickel-titanium alloycomprises no more than about 55 at. % nickel. The nickel-titanium alloymay comprise 50 at. % nickel.

According to a preferred embodiment, the nickel-titanium alloy comprisesat least about 34 at. % titanium. More preferably, the nickel-titaniumalloy comprises at least about 36.5 at. % titanium. Even morepreferably, the nickel-titanium alloy comprises at least about 39 at. %titanium. Still more preferably, the nickel-titanium alloy comprises atleast about 44 at. % titanium.

It is also preferred that the nickel-titanium alloy comprises no morethan about 60 at. % titanium. More preferably, the nickel-titanium alloycomprises no more than about 55 at. % titanium. Even more preferably,the nickel-titanium alloy comprises no more than about 50 at. %titanium.

According to an exemplary embodiment, the nickel-titanium alloycomprises from about 36.5 at. % to about 55 at. % nickel, from about36.5 at. % to about 55 at. % titanium, and from about 2.5 at. % to about12.5 at. % at least one rare earth element. According to anotherexemplary embodiment, the nickel-titanium alloy comprises from about 39at. % to about 55 at. % nickel, from about 39 at. % to about 55 at. %titanium, and from about 5 at. % to about 10 at. % at least one rareearth element.

The nickel-titanium alloy may also contain one or more additionalalloying elements, such as transition metals or other metals. Forexample, one or more of Al, Cr, Mn, Fe, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo,Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg,Tl, Pb, Bi, Po, V, and Mischmetal may be included as the additionalalloying element (AAE). It is preferable that the nickel-titanium alloycomprises no more than about 14.9 at. % AAE. More preferably, thenickel-titanium alloy comprises no more than about 9.9 at. % ME. Evenmore preferably, the nickel-titanium alloy comprises no more than about7.4 at. % AAE. Still more preferably, the nickel-titanium alloycomprises no more than about 4.9 at. % AAE. Most preferably, thenickel-titanium alloy comprises no more than about 1.9 at. % AAE.According to one preferred embodiment, the nickel-titanium alloyincludes at least about 0.1 at. % AAE. Preferably, the additionalalloying element has a lower concentration in the nickel-titanium alloythan the rare earth element when the one or more additional alloyingelements are selected from the group consisting of Ir, Pt, Au, Re, W,Pd, Rh, Ta, Ag, Ru, Hf, Os, Zr, Nb, and Mo.

It is known in the art that equiatomic or near-equiatomic binarynickel-titanium alloys exhibit superelastic or shape memory behavior.Such alloys are commonly referred to as Nitinol or Nitinol alloys.Slightly nickel-rich Nitinol alloys including, for example, 51 at. % Niand 49 at. % Ti, are known to be useful for medical devices which areaustenitic at body temperature. Specifically, alloys including 50.6-50.8at. % Ni and 49.2-49.4 at. % Ti are considered to be medical gradeNitinol alloys.

Accordingly, the nickel-titanium alloy of the present disclosurecomprises about 51 at. % Ni, about 34 at. % Ti, and about 15 at. % RE,according to one preferred embodiment. In another example in which oneor more additional alloying elements (AAE) are present in the alloy, thenickel-titanium alloy preferably includes about 51 at. % Ni, about 34at. % Ti, about (15-x) at. % RE, and about x at. % AAE, where 0≦x≦14.9.Preferably, the rare earth element is in the ternary position in thealloy and 0≦x≦7.4. According to these examples, the rare earth elementsubstitutes for titanium. Alternatively, the rare earth element maysubstitute for nickel, or may substitute for both nickel and titanium.

According to another preferred embodiment, the nickel-titanium alloy ofthe present disclosure comprises about 51 at. % Ni, about 36.5 at. % Ti,and about 12.5 at. % RE. In another example in which one or moreadditional alloying elements (AAE) are present in the alloy, thenickel-titanium alloy preferably includes about 51 at. % Ni, about 36.5at. % ti, about (12.5-x) at. % RE, and about x at. % AAE, where0≦x≦12.4. Preferably, the rare earth element is in the ternary positionin the alloy and 0≦x≦6.2. According to these examples, the rare earthelement substitutes for titanium. Alternatively, the rare earth elementmay substitute for nickel, or may substitute for both nickel andtitanium.

According to another preferred embodiment, the nickel-titanium alloy ofthe present disclosure comprises about 51 at. % Ni, about 39 at. % Ti,and about 10 at. % RE. In another example in which one or moreadditional alloying elements (AAE) are present in the alloy, thenickel-titanium alloy preferably includes about 51 at. % Ni, about 39at. % Ti, about (10-x) at. % RE, and about x at. % AAE, where 0≦x≦9.9.Preferably, the rare earth element is in the ternary position in thealloy and 0≦x≦4.9. According to these examples, the rare earth elementsubstitutes for titanium. Alternatively, the rare earth element maysubstitute for nickel, or may substitute for both nickel and titanium.

According to another preferred embodiment, the nickel-titanium alloycomprises about 51 at. % Ni, about 41.5 at. % Ti, and about 7.5 at. %RE. In another example in which one or more additional alloying elementsare present in the alloy, the nickel-titanium alloy preferably includesabout 51 at. % Ni, about 41.5 at. % Ti, about (7.5-x) at. % RE, andabout x at. % AAE, where 0≦x≦7.4. Preferably, the rare earth element isin the ternary position in the alloy and 0≦x≦3.7. According to theseexamples, the rare earth element substitutes for titanium.Alternatively, the rare earth element may substitute for nickel, or maysubstitute for both nickel and titanium.

According to another preferred embodiment, the nickel-titanium alloycomprises about 51 at. % Ni, about 44 at. % Ti, and about 5.0 at. % RE.In another example in which one or more additional alloying elements arepresent in the alloy, the nickel-titanium alloy includes about 51 at. %Ni, about 44 at. % Ti, about (5.0-x) at. % RE, and about x at. % AAE,where 0≦x≦4.9. Preferably, the rare earth element is in the ternaryposition in the alloy and 0≦x≦2.4. According to these examples, the rareearth element substitutes for titanium. Alternatively, the rare earthelement may substitute for nickel, or may substitute for both nickel andtitanium.

According to another preferred embodiment, the nickel-titanium alloycomprises about 51 at. % Ni, about 46.5 at. % Ti, and about 2.5 at. %RE. In another example in which one or more additional alloying elementsare present in the alloy, the nickel-titanium alloy includes about 51at. % Ni, about 46.5 at. % Ti, about (2.5-x) at. % RE, and about x at. %AAE, where 0≦x≦2.4. Preferably, the rare earth element is in the ternaryposition in the alloy and 0≦x≦1.2. According to these examples, the rareearth element substitutes for titanium. Alternatively, the rare earthelement may substitute for nickel, or may substitute for both nickel andtitanium.

In an alternative embodiment, the nickel-titanium alloy may includeabout 50 at. % Ni, (50-y-x) at. % Ti, y at. % RE, and x at. % AAE, wherex is no more than about 15 and y is no more than about 14.9, asdescribed previously. In another example, the nickel-titanium alloy mayinclude about 52 at. % Ni, (48-y-x) at. % Ti, y at. % RE, and x at. %AAE, with x and y having the bounds described above. Alternatively, thealloy may include about 53 at. % Ni, (47-y-x) at. % Ti, y at. % RE, andx at. % AAE. It is also envisioned that the alloy may include about 54at. % Ni, (46-y-x) at. % Ti, y at. % RE, and x at. % AAE, or 55 at. %Ni, (45-y-x) at. % Ti, y at. % RE, and x at. % AAE. In another example,the alloy may include about or 56 at. % Ni, (44-y-x) at. % Ti, y at. %RE, and x at. % AAE. According to one preferred embodiment, y is equalto (4-x), and x has the exemplary values shown in Table 1 below.

TABLE 1 Preferred Alloy Compositions (at. %) Ni₅₀Ti₄₆RE₄Ni₅₀Ti₄₆RE_(3.75)AAE_(0.25) Ni₅₀Ti₄₆RE_(3.5)AAE_(0.5)Ni₅₀Ti₄₆RE_(3.25)AAE_(0.75) Ni₅₀Ti₄₆RE₃AAE₁ Ni₅₁Ti₄₅RE₄Ni₅₁Ti₄₅RE_(3.75)AAE_(0.25) Ni₅₁Ti₄₅RE_(3.5)AAE_(0.5)Ni₅₁Ti₄₅RE_(3.25)AAE_(0.75) Ni₅₁Ti₄₅RE₃AAE₁ Ni₅₂Ti₄₄RE₄Ni₅₂Ti₄₄RE_(3.75)AAE_(0.25) Ni₅₂Ti₄₄RE_(3.5)AAE_(0.5)Ni₅₂Ti₄₄RE_(3.25)AAE_(0.75) Ni₅₂Ti₄₄RE₃AAE₁ Ni₅₃Ti₄₃RE₄Ni₅₃Ti₄₃RE_(3.75)AAE_(0.25) Ni₅₃Ti₄₃RE_(3.5)AAE_(0.5)Ni₅₃Ti₄₃RE_(3.25)AAE_(0.75) Ni₅₃Ti₄₃RE₃AAE₁ Ni₅₄Ti₄₂RE₄Ni₅₄Ti₄₂RE_(3.75)AAE_(0.25) Ni₅₄Ti₄₂RE_(3.5)AAE_(0.5)Ni₅₄Ti₄₂RE_(3.25)AAE_(0.75) Ni₅₄Ti₄₂RE₃AAE₁ Ni₅₅Ti₄₁RE₄Ni₅₅Ti₄₁RE_(3.75)AAE_(0.25) Ni₅₅Ti₄₁RE_(3.5)AAE_(0.5)Ni₅₅Ti₄₁RE_(3.25)AAE_(0.75) Ni₅₅Ti₄₁RE₃AAE₁ Ni₅₆Ti₄₀RE₄Ni₅₆Ti₄₀RE_(3.75)AAE_(0.25) Ni₅₆Ti₄₀RE_(3.5)AAE_(0.5)Ni₅₆Ti₄₀RE_(3.25)AAE_(0.75) Ni₅₆Ti₄₀RE₃AAE₁

Erbium (Er) is a preferred rare earth element. It is,believed that Er isless likely to cause cracking or brittleness of the nickel-titaniumalloy at increasing rare earth concentrations than other rare earthelements. Chromium (Cr) is a preferred additional alloying element(AAE). It is believed that increasing concentrations of chromium areeffective for suppressing the austenitic phase transformationtemperatures of the alloy to near body temperature, as further discussedbelow. Nickel-rich alloys are also known to have suppressedtransformation temperatures. Accordingly, compiled in Table 2 below areseveral preferred Ni—Ti alloy compositions that include Er and Cr, alongwith increasing concentrations of nickel.

TABLE 2 Preferred Alloy Compositions including Er and Cr (at. %)Ni₅₀Ti₄₆Er₄ Ni₅₀Ti₄₆Er_(3.75)Cr_(0.25) Ni₅₀Ti₄₆Er_(3.5)Cr_(0.5)Ni₅₀Ti₄₆Er_(3.25)Cr_(0.75) Ni₅₀Ti₄₆Er₃Cr₁ Ni₅₁Ti₄₅Er₄Ni₅₁Ti₄₅Er_(3.75)Cr_(0.25) Ni₅₁Ti₄₅Er_(3.5)Cr_(0.5)Ni₅₁Ti₄₅Er_(3.25)Cr_(0.75) Ni₅₁Ti₄₅Er₃Cr₁ Ni₅₂Ti₄₄Er₄Ni₅₂Ti₄₄Er_(3.75)Cr_(0.25) Ni₅₂Ti₄₄Er_(3.5)Cr_(0.5)Ni₅₂Ti₄₄Er_(3.25)Cr_(0.75) Ni₅₂Ti₄₄Er₃Cr₁ Ni₅₃Ti₄₃Er₄Ni₅₃Ti₄₃Er_(3.75)Cr_(0.25) Ni₅₃Ti₄₃Er_(3.5)Cr_(0.5)Ni₅₃Ti₄₃Er_(3.25)Cr_(0.75) Ni₅₃Ti₄₃Er₃Cr₁ Ni₅₄Ti₄₂Er₄Ni₅₄Ti₄₂Er_(3.75)Cr_(0.25) Ni₅₄Ti₄₂Er_(3.5)Cr_(0.5)Ni₅₄Ti₄₂Er_(3.25)Cr_(0.75) Ni₅₄Ti₄₂Er₃Cr₁ Ni₅₅Ti₄₁Er₄Ni₅₅Ti₄₁Er_(3.75)Cr_(0.25) Ni₅₅Ti₄₁Er_(3.5)Cr_(0.5)Ni₅₅Ti₄₁Er_(3.25)Cr_(0.75) Ni₅₅Ti₄₁Er₃Cr₁ Ni₅₆Ti₄₀Er₄Ni₅₆Ti₄₀Er_(3.75)Cr_(0.25) Ni₅₆Ti₄₀Er_(3.5)Cr_(0.5)Ni₅₆Ti₄₀Er_(3.25)Cr_(0.75) Ni₅₆Ti₄₀Er₃Cr₁

Palladium (Pd) may also be useful for suppressing the austenitic phasetransformation temperature of the alloy, and it may further improve theradiopacity of the material. Accordingly, Pd may be included as analloying element in place of or in addition to Cr. It may also be usefulto include iron (Fe) in the alloy composition as a quaternary or higherorder elemental addition, as iron can improve the hot workability of thenickel-titanium alloy.

Small amounts (e.g., hundreds of ppm) of non-metal elemental additions,such as, for example, C, H, N, or O, may also be present in thenickel-titanium alloy, although non-metallic elements are generally notincluded in the summation of alloying elements used to specify thecomposition of the alloy. Preferably, the amounts of C, O, and N areconsistent with the American Society of Testing and Materials (ASTM)standard F2063, so as to avoid forming a high number density of and/orlarge-size carbide, oxide, nitride or complex carbonitride particles.This may result in a better electropolished surface and better fatiguelife of the nickel-titanium alloy. H is preferably controlled per ASTMstandard F2063 to minimize hydrogen embrittlement of the alloy. Theaforementioned ASTM standards are hereby incorporated by reference.

The nickel-titanium alloy has a phase structure that depends on thecomposition and processing history of the alloy. The rare earth elementmay form a solid solution with nickel and/or titanium. The rare earthelement may also form one or more binary intermetallic compound phaseswith nickel and/or with titanium. In other words, the rare earth elementmay combine with nickel in specific proportions and/or with titanium inspecific proportions. Without wishing to be bound by theory, it isbelieved that most of the rare earth elements set forth as preferredternary alloying additions will substitute for titanium and form one ormore intermetallic compound phases with nickel, such as, for example,NiRE, Ni₂RE, Ni₃RE₂ or Ni₃RE₇. In some cases, however, the rare earthelement may substitute for nickel and combine with titanium to form asolid solution or a compound such as Ti_(x)RE_(y). The nickel-titaniumalloy may also include one or more other intermetallic compound phasesof nickel and titanium, such as NiTi, Ni₃Ti and/or NiTi₂, depending onthe composition and heat treatment. The rare earth addition may form aternary intermetallic compound phase with both nickel and titaniumatoms, such as Ni_(x)Ti_(y)RE_(z). Some exemplary phases in variousNi—Ti-RE alloys are identified below in Table 3. Also, in the event thatone or more additional alloying elements are present in thenickel-titanium alloy, the additional alloying elements may formintermetallic compound phases with nickel, titanium, and/or the rareearth element.

TABLE 3 Exemplary Phases in Ni—Ti-RE Alloys Alloy Exemplary PhasesNi—Ti—Dy DyNi, DyNi₂, Dy_(x)Ti_(y), α(Ti), α(Ni), Ni_(x)Ti_(y)Dy_(z)Ni—Ti—Er ErNi, ErNi₂, Er_(x)Ti_(y), α(Ti), α(Ni), Ni_(x)Ti_(y)Er_(z)Ni—Ti—Gd GdNi, GdNi₂, Gd_(x)Ti_(y), α(Ti), α(Ni), Ni_(x)Ti_(y)Gd_(z)Ni—Ti—La LaNi, La₂Ni₃, La_(x)Ti_(y), α(Ti), α(Ni), Ni_(x)Ti_(y)La_(z)Ni—Ti—Nd NdNi, NdNi₂, Nd_(x)Ti_(y), α(Ti), α(Ni), Ni_(x)Ti_(y)Nd_(z)Ni—Ti—Yb YbNi₂, Yb_(x)Ti_(y), α(Ti), α(Ni), Ni_(x)Ti_(y)Yb_(z)

The phase structure of the nickel-titanium alloy may be determined byexperimental and/or computational methods. For example, diffractionmethods, such as x-ray diffraction, neutron diffraction, and/or electrondiffraction, may be employed. Alternatively, the CALPHAD method(CALculation of PHAse Diagrams) may be employed. Implementation of theCALPHAD method is discussed in “Thermodynamic Modeling of MulticomponentPhase Equilibria,” JOM 49, 12 (1997) 14-19, which is hereby incorporatedby reference. A number of commercially available software programs maybe used to carry out the CALPHAD method, including, for example,ChemSage, MTDATA and Thermo-Calc. The Thermo-Calc program, for example,uses a combination of pre-existing published data on elements and dataprovided by the user in order to calculate phase diagrams. The programincludes some pre-existing data for NiTi, while data and thermodynamicequations for rare earth systems obtained from the scientific literaturemay have to be provided. A ternary phase diagram can be constructed fromthese two sets of information. The process involves entering the knownphase data, adding additional phases unknown to the program, andmanipulating the interactions between the elements and phases. A set ofequations derived from these manipulations may then be applied toinvariant points or features of the phase diagram which are known orexpected, and the program calculates the diagram from the given data,optimizing the given parameters to fit.

Ab initio superstructure calculations may be used to determine theenergetics of the substitution mechanisms, that is, whether the rareearth element is substituting for nickel or titanium. These calculationsalso reveal the effect of the rare earth substitution on the mechanicalproperties of the energetically favorable configurations. Once theenergetics of the alloys of interest are determined, semi-empiricalinteratomic potentials may be fit to the ab initio data and to availableexperimental data to describe the alloys. For example, these potentialmodels may be utilized to predict and describe the dynamic behaviour ofthe nickel-titanium alloys, e.g., the dependence of phase stability ontemperature and pressure (stress), which may be indicative of the M_(f)and A_(f) temperatures.

In selecting a desired alloy composition, the effect of the rare earthalloying element on various properties of the nickel-titanium alloy,including radiopacity, transformation temperatures (M_(f), M_(s),R′_(s), R′_(f), R_(s), R_(f), A_(s), A_(f)), and mechanical properties,may be considered.

The radiopacity of a material is related to its linear absorptioncoefficient, μ, which depends on its effective atomic number (Z_(eff))and density (ρ), and on the energy (E) of the incoming x-ray photons:

$\frac{\mu}{\rho} = {{const} \cdot \frac{Z_{eff}^{3}}{E^{3}}}$

The linear absorption coefficient μ is proportional to the density ρ ofthe material, and thus the quantity

$\frac{\mu}{\rho}$

is a material constant known as the mass absorption coefficient andexpressed in units of cm² g⁻¹.

Linear absorption coefficients μ were calculated for several rare earthelements and also for platinum for comparison. The results are shown inFIG. 2. In FIG. 3, the linear absorption coefficients μ are shownnormalized with respect to the linear absorption coefficient of platinumμ_(Pt). The figures indicate that the absorption of the rare earthelements tends to peak in the photon energy range of about 40 to 80 keV,with some rare earth elements exceeding the absorption of platinum inthis region.

Linear absorption coefficients were also calculated for several Ni—Ti-REalloy compositions, as will be described below. The calculations werecarried out under simulated x-ray conditions in order to evaluate thepotential for rare earth alloying additions to improve the radiopacityof nickel-titanium medical devices.

To carry out a typical diagnostic x-ray procedure, a x-ray source ortube may be disposed in opposition to a patient with at least one filterplaced between the source and the patient. A diagnostic x-ray tubetypically has a built-in aluminum filter of about 2.5 mm in thicknessper Food and Drug Administration (FDA) regulations, and additionalfilters may be used to achieve further filtering of the emitted x-raybeam. The x-ray photons may be generated when electrons from a tungstenfilament are accelerated by a tube voltage and bombard a W or W/Re anodewithin the x-ray tube. Typically, for diagnostic procedures, the tubevoltage is in the range of from about 50 kVp to about 150 kVp. Thex-rays generated by the bombardment may pass through a beryllium windowand through the one or more filters disposed between the source and thepatient. The x-rays also experience a filtering or attenuation effectwhen passing through air and through tissues of the patient.

The x-ray beam emitted from the x-ray tube is not monochromatic, butrather includes a distribution of photons over a range of energies.Referring to FIGS. 4A-4D, the x-ray photons have a maximum energycorresponding to the tube voltage. For example, at a tube voltage of 70kVp (see FIG. 4B), the maximum energy of the x-ray beam is 70 keV. Thex-ray beam has a peak intensity (maximum number of photons) at an energycorresponding to about one-third of the maximum photon energy. The peakintensity may be shifted to higher energies, however, by the use of oneor more filters. Other attenuation effects, such as passage of the x-raybeam through body tissue, may also cause a shift of the maximumintensity to higher energies, a phenomenon that may be referred to asbeam hardening. For example, as shown in FIG. 4B, the peak intensity ofthe x-ray beam may be shifted to about 45 keV from about 35 keV byincluding a 0.2 mm copper filter in addition to a 2.5 mm aluminum filterbetween the x-ray source and the patient. By replacing the 0.2 mm copperfilter with a 0.3 mm copper filter, the peak intensity of the x-ray beammay be shifted to about 50 keV. Generally speaking, the one or morefilters may cause a shift of between 5 keV and 30 keV in the peakintensity of the radiation passing through the filter.

The intensity of x-rays transmitted through a material I_(x) is relatedto the incident intensity I₀, material thickness x, and the linearabsorption coefficient μ:

I_(x)=I₀e^(−μx)

Materials or tissues that substantially transmit incident x-rays are notreadily visible in x-ray images. In contrast, radiopaque materialsabsorb incident x-rays over a given energy range and tend to show highcontrast and good visibility in x-ray images. The magnitude of thelinear absorption coefficient of a material may be a good indicator ofits capacity for absorbing x-ray radiation, and thus its radiopacity.

Linear absorption coefficients were calculated for several Ni—Ti-REalloy compositions using a software program called XMuDat developed byRobert Nowotny of the Institut f. Biomed. Technik und Physik at theUniversity of Wien, Wien, Austria. XMuDat is a computer program for thepresentation and calculation of various photon interaction coefficientsfor materials of dosimetric interest. Data for mass attenuation-, massenergy transfer- and mass energy absorption coefficients in a photonenergy range of 1 keV to 50 MeV are available. For calculation theprogram uses photon interaction coefficients collected from J M. Boone,A E. Chavez; Medical Physics 23, 12 (1996) 1997-2005.

The effects of various diagnostic x-ray tube voltages and filtrationschemes were considered, as summarized in Table 4 below. The raw datafor unfiltered photons at various tube voltages were taken from HorstAichinger, Joachim Dierker, Sigrid Joite-Barfuβ and Manfred Säbel,Radiation Exposure and Image Quality in X-Ray Diagnostic Radiology:Physical Principles and Clinical Applications, Springer: Berlin. Thepolychromatic nature of the x-ray beam generated from a W/Re anode andthe role of beam attenuation with various filters were also taken intoaccount.

TABLE 4 Parameters of Linear Absorption Coefficient CalculationsParameter Range Considered Tube voltage 40 kVp-125 kVp FiltrationUnfiltered 2.5 mm Al 2.5 mm Al + 0.1 mm Cu 2.5 mm Al + 0.2 mm Cu 2.5 mmAl + 0.3 mm Cu Rare earth (RE) addition Dy, Eu, Gd, La, Nd, Sm, TbConcentration of RE addition 2.5, 5, 7.5, 10, 12.5, and 15 at. %

As a first step in the calculations, mass absorption coefficientsA_(alloy) for various alloy compositions were calculated using a rule ofmixtures approach:

A_(alloy) = p A_(Ni) = q A_(Ti) + r A_(RE) where$p = \frac{a\; M_{Ni}}{\left( {{a\; M_{Ni}} + {b\; M_{Ti}} + {c\; M_{RE}}} \right)}$$q = \frac{b\; M_{Ni}}{\left( {{a\; M_{Ni}} + {b\; M_{Ti}} + {c\; M_{RE}}} \right)}$$r = \frac{c\; M_{RE}}{\left( {{a\; M_{Ni}} + {b\; M_{Ti}} + {c\; M_{RE}}} \right)}$

The variables A_(Ni), A_(Ti), and A_(RE) represent elemental massabsorption coefficients, which are equivalent to

$\frac{\mu}{\rho}$

for each element. The variables M_(Ti), M_(Ni), and M_(RE) represent themolecular weight of each element and a, b, and c are atomic percentagesof each element in the alloy. It was assumed in estimating the atomicpercentages that the rare earth element substituted for titanium. Thisassumption was made based on the closer proximity of the rare earthelements to titanium than to nickel in the periodic table. Since theradiopacity of nickel is comparable to the radiopacity of titanium inthe energy range of interest for diagnostic x-ray procedures, thespecifics of the substitution are believed to be less important than theatomic percentage of the rare earth element in the Ni—Ti-RE alloy.

Once the mass absorption coefficient A_(alloy) was obtained for a givenalloy composition, the linear absorption coefficient μ_(alloy) wascalculated as the product of A_(alloy) and the density ρ_(alloy) of thealloy. The density ρ_(alloy) was calculated using the same rule ofmixtures approach as above.

Next, a cumulative linear absorption coefficient μ_(alloy) ^(C) wascalculated for each alloy composition to take into account thepolychromatic nature of the x-ray beam. Using x-ray intensitydistributions for a W/Re anode at various x-ray tube voltages and withdifferent levels of filtration, photon probability distributions werecalculated. Cumulative linear absorption coefficients μ_(alloy) ^(C)were obtained for various tube voltages and filtration levels bymultiplying the values of μ_(alloy) determined above by the respectivephoton probability at a given energy and then summing the values overthe entire energy spectrum. The resulting values of μ_(alloy) ^(C), orradiopacity, are shown in graphical form in FIGS. 5-8 for variousNi—Ti-RE alloy compositions in atomic percent (at. %), tube voltages andfiltration schemes. Calculated data are also presented for Ni—Ti—Pt,Ni—Ti—Pd, and Ni—Ti—W alloys for comparison.

It is desirable that the Ni—Ti-RE alloys exhibit improved radiopacitycompared to a binary Nitinol alloy. Therefore, the cumulative linearabsorption coefficients μ_(alloy) ^(C) obtained for various Ni—Ti-REalloy compositions were normalized to the cumulative linear absorptioncoefficient μ_(NiTi) ^(C) of binary Nitinol, thus obtaining values ofrelative radiopacity R_(rel), i.e.,

$\left( {R_{rel} = \frac{\mu_{alloy}^{C}}{\mu_{NiTi}^{C}}} \right).$

A slightly nickel-rich composition of 50.6 at. % Ni was assumed incalculating μ_(NiTi) ^(C) for binary Nitinol. Using this approach, it ispossible to compare the radiopacity of the Ni—Ti-RE alloys to theradiopacity of a near-equiatomic binary Ni—Ti alloy. The relativeradiopacity values R_(rel) are shown in graphical form in FIGS. 9-12 forvarious Ni—Ti-RE alloy compositions in atomic percent (at. %), tubevoltages and filtration schemes (e.g., unfiltered, Al filter, Cu filter,or CDRH phantom, which is described later and shown in FIG. 20).Calculated data are also presented for Ni—Ti—Pt, Ni—Ti—Pd, and Ni—Ti—Walloys for comparison.

Referring to the calculated data shown in FIGS. 9-12, it can be observedthat the radiopacity of the Ni—Ti-RE alloys is greater than that of anear-equiatomic binary nickel-titanium alloy. The Ni—Ti-RE alloys have acumulative absorption coefficient μ_(alloy) ^(C) (radiopacity) rangingfrom greater than about 1 to about 3.2 times that of a near-equiatomicbinary nickel-titanium alloy when exposed to radiation having an energyin the range of from 15 keV to 125 keV. This is shown, for example, inFIG. 12, which corresponds to a tube voltage of 125 kVp. The Ni—Ti-REalloys have a cumulative absorption coefficient μ_(alloy) ^(C)(radiopacity) ranging from greater than about 1 to about 2.7 times thatof a near-equiatomic binary nickel-titanium alloy when exposed toradiation having an energy in the range of from 15 keV to 80 keV, asshown, for example, in FIG. 11A, which corresponds to a tube voltage of80 kVp. The Ni—Ti-RE alloys have a cumulative absorption coefficientμ_(alloy) ^(C) (radiopacity) ranging from greater than about 1 to about2.5 times that of a near-equiatomic binary nickel-titanium alloy whenexposed to radiation having an energy in the range of from 15 keV to 70keV, as shown, for example, in FIG. 10, which corresponds to a tubevoltage of 70 kVp.

By using more than one rare earth element and/or additional alloyingelements in the nickel-titanium alloy, the radiopacity may be increasedin a cumulative manner consistent with the radiopacity of the individualalloying elements.

Preferably, the nickel-titanium alloy has a radiopacity in the range offrom greater than about 1 to about 8 times that of a near-equiatomicbinary nickel-titanium alloy (i.e., the relative radiopacity R_(rel) isin the range of from about 1 to about 8) when exposed to radiationhaving an energy in the range of from 15 keV to 150 keV. The radiopacityof the nickel-titanium alloy may also be in the range of from greaterthan about 1 to about 8 times that of a near-equiatomic binarynickel-titanium alloy when the alloys are exposed to radiation in therange of from 15 keV to 125 keV. According to other embodiments, theradiopacity may be in the range of from greater than about 1 to about 8times that of a near-equiatomic binary nickel-titanium alloy when thealloys are exposed to radiation in the range of from 15 keV to 80 keV,from 15 keV to 70 keV, or from 15 keV to 60 keV.

More preferably, the nickel-titanium alloy has a radiopacity in therange of from about 1.2 to about 8 times that of a near-equiatomicbinary nickel-titanium alloy (i.e., the relative radiopacity R_(rel) isin the range of from about 1.2 to about 8) when exposed to radiationhaving an energy in the range of from 15 keV to 150 keV. The radiopacityof the nickel-titanium alloy may also be in the range of from about 1.2to about 8 times that of a near-equiatomic binary nickel-titanium alloywhen the alloys are exposed to radiation in the range of from 15 keV to125 keV. According to other embodiments, the radiopacity may be in therange of from about 1.2 to about 8 times that of a near-equiatomicbinary nickel-titanium alloy when the alloys are exposed to radiation inthe range of from 15 keV to 80 keV, from 15 keV to 70 keV, or from 15keV to 60 keV.

Even more preferably, the nickel-titanium alloy has a radiopacity in therange of from about 1.2 to about 5 times that of a near-equiatomicbinary nickel-titanium alloy (i.e., the relative radiopacity R_(rel) isin the range of from about 1.2 to about 5) when exposed to radiationhaving an energy in the range of from 15 keV to 150 keV. The radiopacityof the nickel-titanium alloy may also be in the range of from about 1.2to about 5 times that of a near-equiatomic binary nickel-titanium alloywhen the alloys are exposed to radiation in the range of from 15 keV to125 keV. According to other embodiments, the radiopacity may be in therange of from about 1.2 to about 5 times that of a near-equiatomicbinary nickel-titanium alloy when the alloys are exposed to radiation inthe range of from 15 keV to 80 keV, from 15 keV to 70 keV, or from 15keV to 60 keV.

It may be even more advantageous if the radiopacity of thenickel-titanium alloy is in the range of from about 1.5 to about 5 timesgreater than that of a near-equiatomic binary nickel-titanium alloy whenthe alloys are exposed to radiation having an energy within any of theabove-mentioned ranges (i.e., from 15 keV to 150 keV, from 15 keV to 125keV, from 15 keV to 80 keV, from 15 keV to 70 keV, or from 15 keV to 60keV).

According to a preferred embodiment, the nickel-titanium alloy has aradiopacity in the range of from greater than about 1 to about 8 timesthat of a near-equiatomic binary nickel-titanium alloy when exposed toradiation having a peak intensity at an energy in the range of from 30keV to 60 keV. It is also preferable that the radiopacity of thenickel-titanium alloy is in the range of from greater than about 1 toabout 8 times that of a near-equiatomic binary nickel-titanium alloywhen exposed to radiation having a peak intensity at an energy in therange of from 35 keV to 55 keV, or from 40 keV to 50 keV.

According to another preferred embodiment, the nickel-titanium alloy hasa radiopacity in the range of from greater than about 1.2 to about 5times that of a near-equiatomic binary nickel-titanium alloy whenexposed to radiation having a peak intensity at an energy in the rangeof from 30 keV to 60 keV. It is also preferable that the radiopacity ofthe nickel-titanium alloy is in the range of from greater than about 1.2to about 5 times that of a near-equiatomic binary nickel-titanium alloywhen exposed to radiation having a peak intensity at an energy in therange of from 35 keV to 55 keV, or from 40 keV to 50 keV.

Again referring to the calculated data shown in FIGS. 9-12, theradiopacity of the Ni—Ti-RE alloys is comparable to or better than thatof Ni—Ti—Pd at a tube voltage in the range of from 70 kVp to 125 kVp,depending on the filter selection. FIGS. 9-12 correspond to tubevoltages of 40 kVp, 70 kVp, 80 kVp, and 125 kVp, respectively. Referringto FIG. 11A, for example, a nickel-titanium alloy including 7.5 at. % Ndhas a relative radiopacity R_(rel) of approximately 1.9 when a 2.5 mm Alfilter and 0.3 mm Cu filter are used, whereas a nickel-titanium alloyincluding 7.5 at. % Pd has a relative radiopacity R_(rel) of about 1.7under the same conditions. Preferably, the radiopacity of the Ni—Ti-REalloys is comparable to or better than that of Ni—Ti—Pd at a tubevoltage in the range of from 60 kVp to 150 kVp.

It also can be observed from the calculated data that the radiopacity ofthe Ni—Ti-RE alloys increases at higher concentrations of the rare earthalloying addition. Referring again to FIG. 11A, for example, the maximumradiopacity (largest value of μ_(rel) ^(C)) for each alloy compositionis achieved at the highest rare earth element concentration (15 at. %)considered in the calculations.

In addition to considering the impact of the rare earth element(s) onthe radiopacity of the nickel-titanium alloy, it is also desirable toconsider the impact on the superelastic and mechanical properties of thealloy. The improved radiopacity achieved at high concentrations of rareearth elements preferably may be balanced against the effects of highconcentrations of alloying elements on the superelastic and mechanicalproperties of the nickel-titanium alloy.

According to a preferred embodiment, the nickel-titanium alloy exhibitssuperelastic or shape memory behavior. That is, the nickel-titaniumalloy undergoes a reversible phase transformation that allows it to“remember” and return to a previous shape or configuration. Thenickel-titanium alloy transforms between a lower temperature phase(martensite) and a higher temperature phase (austenite). Austenite ischaracteristically the stronger phase, and martensite may be deformed upto a recoverable strain of about 8%. Strain introduced in the alloy inthe martensitic phase to achieve a shape change may be substantiallyrecovered upon completion of a reverse phase transformation toaustenite, allowing the alloy to return to a previous shape. The strainrecovery may be driven by the application and removal of stress(superelastic effect) and/or by a change in temperature (shape memoryeffect).

The stress-strain diagram in FIG. 13 illustrates the superelastic effectfor an exemplary nickel-titanium alloy at a temperature above theaustenitic final temperature (A_(f)) of the alloy. Upon application of astress σ_(a), an alloy in a first configuration begins to transform fromaustenite to martensite as a result of the formation of stress-inducedmartensite. The martensitic phase of the alloy can accommodate severalpercent strain at a nearly constant stress. At a stress of σ_(b), whichcorresponds to 8% strain in this example, the martensitic transformationis complete and the alloy has been deformed to a second configuration.Upon release of the stress, the martensite begins to transform back toaustenite and the alloy recovers the strain at a lower plateau stress ofσ_(c). The nickel-titanium alloy thus returns to the firstconfiguration.

FIG. 14 shows a typical transformation temperature curve for anexemplary nickel-titanium shape memory alloy, where the y-axisrepresents the amount of martensite in the alloy and the x-axisrepresents temperature. At or above a temperature of A_(f), thenickel-titanium alloy has a fully austenitic structure. Following thearrows, the alloy may be cooled to a temperature of M_(s), at whichpoint the transformation to the martensitic phase begins. Furthercooling leads to an increase in the percentage of martensite in thematerial, ultimately leading to a fully martensitic structure at atemperature of M_(f), as shown in FIG. 14.

Now referring also to FIG. 15, which shows strain versus temperature foran exemplary nickel-titanium shape memory alloy, the fully martensiticstructure attained at a temperature of M_(f) may be strained from afirst configuration to a second configuration (as shown by the stresssymbol σ). The alloy may accommodate several percent recoverable strain(8% in this example). To reverse the phase transformation and recoverthe strain, the temperature of the alloy is increased. Again followingthe arrows, the nickel-titanium alloy may be warmed to a temperature ofA_(s), at which point the alloy begins to transform to the austeniticphase. Upon further heating, the transformation to austenite progressesand the alloy gradually recovers the first configuration. Ultimately, ata temperature of A_(f) or higher, the material has completed the returntransformation to the austenitic phase (0% martensite) and has fullyrecovered the 8% strain.

According to one embodiment, the nickel-titanium alloy may include anintermediate temperature R-phase in addition to the higher temperatureaustenitic phase and the lower temperature martensitic phase. In otherwords, the R-phase may appear prior to martensite upon cooling fromaustenite. Similarly, the R-phase may appear prior to austenite uponheating from martensite. Whether or not the nickel-titanium alloyincludes the R-phase depends on the composition and processing historyof the alloy.

For the purposes of this disclosure, a nickel-titanium alloy thatprovides a substantial amount of recoverable strain (i.e., an elasticstrain of at least about 0.5%) upon the removal of a deforming stressmay be referred to as a superelastic alloy, whether or not the behavioris driven by phase transformations between martensite and austenite. Forexample, a recoverable strain of about 0.75% may be obtained by stress-and/or temperature-induced phase transformations between austenite andthe R-phase (Using Nitinol Alloys, Johnson Mathey, San Jose, Calif.(2004) p. 17). It is also known that cold-worked martensiticnickel-titanium alloys can provide a recoverable strain of severalpercent (e.g., 3-4%) without a phase transformation to austenite(Duerig, T. W. et al., Linear Superelasticity in Cold-Worked Ni—Ti,Engineering Aspects of Shape Memory Alloys, Butterworth-Heinemann Ltd.,London (1990) pp. 414-419). Preferably, the nickel-titanium alloy of thepresent disclosure provides a recoverable strain in the range of fromabout 0.5% to about 10%. More preferably, the recoverable strain is inthe range of from about 2% to about 10%. Even more preferably, therecoverable strain is in the range of from about 3% to about 10%. Mostpreferably, the recoverable strain is in the range of from about 5% toabout 10%.

Preferably, the medical device includes at least one componentcomprising the nickel-titanium alloy described herein. The component maybe formed in whole or in part of the nickel-titanium alloy from wire,tubing, ribbon, button, bar, disk, sheet, foil, or another cast orworked shape. According to one embodiment, the component has a compositestructure in which one or more portions of the structure are formed ofthe Ni—Ti-RE alloy, and one or more portions of the structure are formedof a different material. For example, the component may include distinctconstituents, such as layers, cladding, filaments, strands, cables,particles, fibers, and/or phases, where one or more of the constituentsare formed from the Ni—Ti-RE alloy, and one or more are formed from thedifferent material. The different material may be a near-equiatomicbinary nickel-titanium alloy, according to one embodiment, or a materialincluding one or more elements selected from the group consisting of:Al, Cr, Mn, Fe, Co, Cu, Zn, Ga, Ge, Tc, Cd, In, Sn, Sb, Hg, Tl, Pb, Bi,Po, V, Ir, Pt, Au, Re, W, Pd, Rh, Ta, Ag, Ru, Hf, Os, Zr, Nb, and Mo.Such a composite structure may provide a component having improvedradiopacity and optimized superelastic and/or mechanical propertiescompared to a monolithic component.

The component including the nickel-titanium alloy described herein mayinclude at least one wire. The wire may have a composite structureincluding, for example, a core layer and one or more outer layersdisposed about the core layer. Preferably, one or more of the layers areformed of the Ni—Ti-RE alloy. One or more of the layers may be formed ofa different material. The different material may be a binarynickel-titanium alloy or a material including one or more of theelements mentioned above. According to the embodiment shown in FIG. 16,the wire 1600 may include a core layer 1610 made of the Ni—Ti-RE alloyand an outer layer 1620 made of a near-equiatomic binary nickel-titaniumalloy. Alternatively, the core layer 1610 may be made of thenear-equiatomic binary nickel-titanium alloy and the outer layer 1620may be made of the Ni—Ti-RE alloy. The wire 1600 may be formed by, forexample, drawing or extruding a perform including multiple coaxiallayers to form the composite structure. Alternatively, the wire 1600 maybe formed by coating one or more layers on a core layer by plating oranother deposition technique.

The component may include two, three, four, five, six, or more wires,according to one embodiment, where each wire is made in whole or in partof the nickel-titanium alloy of the present disclosure. It is alsocontemplated that one or more of the wires may be made in whole or inpart of a different material, such as a near-equiatomic binarynickel-titanium alloy or a radiopaque metal. Referring to FIGS. 17A and17B, for example, the component may include a plurality of wire strands1700 in a twisted configuration 1710 (e.g., a cable) or a plurality ofwire strands 1700 in a braided configuration 1720, where one or more ofthe strands are made of the Ni—Ti-RE alloy and one or more of thestrands are made of a near-equiatomic binary nickel-titanium alloy.

According to another embodiment, the component comprises a tube or“cannula,” to use terminology common in the medical device community.The cannula may have a composite structure. According to one embodiment,the cannula may be formed from a multilayered tube. Referring forexample to FIG. 18, the cannula 1800 may include one or more coaxiallayers 1810 of Ni—Ti-RE and one or more coaxial layers 1820, 1830 ofanother material, such as a binary nickel-titanium alloy or a radiopaquemetal. The multilayered tube may be formed by drawing or extrudingcoaxial tubing. Alternatively, the multilayered tube may be preparedfrom a clad sheet that has been formed into a tube.

According to another embodiment, the component comprises another cast orworked shape, such as a ribbon, button, bar, rivet, sphere, disk, sheet,or foil.

The above described components may be employed individually or incombination as part of an insertable or implantable medical device, suchas, for example, a stent, a stent graft, a wire guide, a radiopaquemarker or marker band, a torqueable catheter, an introducer sheath, anorthodontic arch wire, or a manipulation, retrieval, or occlusive devicesuch as a grasper, a snare, a basket (e.g., stone extraction ormanipulation basket), a vascular plug, or an embolic protection filter.

According to one embodiment, the device is a stent. All or a portion ofthe stent may be made of the nickel-titanium alloy. The stent mayfurther include a graft material attached thereto. Preferably, the stentis a self-expanding stent. However, balloon-expandable stents may alsobenefit from the Ni—Ti-RE alloy of the present disclosure. The stent maybe formed from one or more wires or cut (e.g., laser cut) from a tube(cannula) using techniques known in the art. The cannula may have acomposite structure as described above. According to another embodimentshown in FIG. 19, the stent 1900 may have a wire structure including oneor more wires. A portion of the wire structure may be formed of Ni—Ti-REand a portion of the wire structure may be formed of a differentmaterial, such as a binary nickel-titanium alloy. The one or more wiresof such a stent may be formed as described above. The stent may furtherinclude a therapeutic surface coating comprising a drug such as, forexample, paclitaxel. The therapeutic surface coating may help toprevent, for example, re-stenosis and the build-up of minerals at thetreatment site.

According to another embodiment, the device is a radiopaque marker ormarker band (“marker”) that provides high x-ray contrast. Such aradiopaque marker may be more readily bonded to a nickel-titaniummedical device than radiopaque markers formed of other materials (e.g.,Pt or Au) due to the similarity between Ni—Ti-RE and binarynickel-titanium. In addition, Ni—Ti-RE radiopaque markers may betterresist galvanic corrosion than other materials when used withnickel-titanium based devices. According to one embodiment, thesuperelastic properties of a Ni—Ti-RE radiopaque marker may aid inattaching the marker to a catheter, stent, wire guide or other medicaldevice. The marker may be designed to fully expand or contract at orabove a temperature corresponding to A_(f) of the Ni—Ti-RE alloy tofacilitate the securing of the marker to the device. For example, aNi—Ti-RE marker band may shrink to fit around a catheter, or a Ni—Ti-REmarker may expand to fit securely within an eyelet of a stent. Ni—Ti-REradiopaque markers may be formed by mechanical working techniques knownin the art, such as swaging, and marker bands may be cut fromthin-walled Ni—Ti-RE tubes.

A method of imaging a medical device within a patient according to thepresent disclosure includes delivering a medical device having at leastone component made from a nickel-titanium alloy including from about 34at. % to about 60 at. % nickel, from about 34 at. % to about 60 at. %titanium, and from about 0.1 at. % to about 15 at. % at least one rareearth element to a site in a patient. The rare earth element is selectedfrom the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Ac, Th, Pa, and U.

The patient is then preferably exposed to radiation having an energy inthe range of from 15 keV to 125 keV to image the medical device. Morepreferably, the energy is in the range of from 15 keV to 80 keV forimaging. Even more preferably, the energy is in the range of from 15 keVto 70 keV, or from 15 keV to 60 keV, for imaging. It is also preferredthat the radiation has a peak intensity at an energy in the range offrom 30 keV to 60 keV. More preferably, the radiation has a peakintensity at an energy in the range of from 35 keV to 55 keV. Even morepreferably, the radiation has a peak intensity at an energy in the rangeof from 40 keV to 50 keV.

To expose the patient to the radiation, the patient may be situated inopposition to an x-ray source with at least one filter disposed betweenthe x-ray source and the patient. The filter may be an aluminum filter(e.g., a 2.5 mm aluminum filter) and/or a copper filter (e.g., a 0.1 mmcopper filter, 0.2 mm copper filter, or a 0.3 mm copper filter), forexample. The x-ray source preferably operates at a voltage (“tubevoltage”) in the range of from 60 kVp to 150 kVp.

A method of using a medical device according to the present disclosureincludes providing a medical device including at least one componentcomprising the nickel-titanium alloy. The medical device (e.g., a stent,stent graft, retrieval device, or an embolic protection filter) may beloaded into a delivery system, according to one aspect of the method.The medical device may then be inserted into a patient and thendelivered to a treatment site in the patient. When positioned at thetreatment site, the device may be deployed. The superelastic and/or theshape memory effect may be used to deliver and deploy the medicaldevice.

According to a preferred embodiment in which the superelastic effect isutilized for delivery and deployment, the device may be maintained in adelivery configuration by a constraining member. For example, aself-expandable stent is typically maintained at a compressed diameterfor delivery within a vessel by a tubular delivery sheath which overliesthe stent. When the constraining member (e.g., the delivery sheath) isremoved and the stress is released, the martensite transforms toaustenite and the medical device may reach (recover) its deployedconfiguration. For example, the self-expandable stent may expand fromthe compressed diameter to an expanded diameter and come into contactwith the vessel wall. The radiopacity of the alloy aids in positioningthe device in the desired location in the body passageway duringdelivery and deployment.

According to this embodiment, the nickel-titanium alloy has an austenitefinish temperature (A_(f)) which is less than or equal to human bodytemperature (37° C.) so that removal of the constraining member issufficient to trigger the transformation to the austenitic phase.Preferably, the A_(f) may be in the range of from about −15° C. to about37° C. Even more preferably, the A_(f) may be in the range of from about−15° C. to about 20° C. An austenite start temperature (A_(s)) of thenickel-titanium alloy is preferably in the range of from about −25° C.to about 20° C., according to one embodiment.

Alternatively, the shape memory effect may be utilized to deliver anddeploy the medical device comprising the nickel-titanium alloy. In otherwords, a change in temperature instead of an applied (removed) stressmay control the transformation from martensite to austenite. Forexample, the stent of the previous example may be deployed by heatinginstead of retraction of a delivery sheath. According to thisembodiment, the nickel-titanium alloy has an austenite finishtemperature (A_(f)) which is less than or equal to body temperature (37°C.). The medical device is maintained at a temperature of less thanA_(f), and preferably less than A_(s), prior to and during delivery ofthe device into the body, thereby maintaining a martensitic structure ofthe nickel-titanium alloy. The device transforms to the austeniticstructure and thus deploys when warmed up to about body temperature.Cooling of the device during delivery is desirable to prevent themartensitic structure from prematurely transforming to austenite. As thedevice is being advanced in the body, the cooling may entail keeping thedevice at a temperature below A_(s) by, for example, flushing a coldfluid through the device or through a delivery system of the device.Preferably, the nickel-titanium alloy has a value of A_(f) of at leastabout 27° C., although an A_(f) of less than about 27° C. is alsopossible. Even more preferably, the nickel-titanium alloy has a value ofA_(f) of at least about 32° C. It is also preferred that A_(f) is nohigher than about 37° C.

In another example utilizing the shape memory effect, the A_(f) of thenickel-titanium alloy is greater than body temperature (37° C.) butbelow a temperature that may be damaging to nearby tissue. Preferably,the A_(f) is at least about 38° C. It is also preferred that the A_(f)is no higher than about 58° C. More preferably, the A_(f) is no higherthan about 50° C. According to this embodiment, the medical device isadvanced through the body to the treatment site without the need forcooling or a constraining member to maintain a martensitic structure.When the device is in place at the treatment site, the device is warmedup to a temperature of A_(f) or higher to transform the martensite toaustenite, and the device deploys to the deployed configuration. Theheating may entail, for example, flushing a warm fluid through themedical device or the delivery system for the device. Once the deployedconfiguration has been obtained, the heating is halted and the deviceremains in the body passageway in the deployed configuration. Tomaintain the austenitic structure of the nickel-titanium alloy while themedical device is in place within the passageway, the nickel-titaniumalloy may be chosen such that M_(f), and preferably M_(s), are belowbody temperature. Since austenite is stronger than martensite, it ispreferable to retain the austenitic phase of the nickel-titanium alloywhen the medical device is deployed. If the martensitic finishtemperature (M_(f)) and the martensitic start temperature (M_(s)) arenot below body temperature, it may be necessary to continuously heat thedevice during deployment to prevent an unwanted phase transformation tomartensite.

The transformation temperatures of the present nickel-titanium alloysmay be adjusted as desired by controlling the composition and processingof the alloys. The transformation temperatures are sensitive to smallchanges in the ratio of nickel to titanium and to the presence of rareearth or other alloying elements. For example, the A_(f) ofstoichiometric NiTi alloys—those having exactly a one-to-one proportionof nickel atoms to titanium atoms—is generally above 100° C., while theA_(f) of a slightly off-stoichiometric alloy including an excess ofnickel (e.g., from about 50.6 to about 50.8 at. % Ni) is generallyaround 0° C. Increasing the proportion of nickel to titanium in thealloy, therefore, provides a means of reducing the A_(f) to the desiredlevel.

The presence of rare earth or other alloying elements also can providean increase or decrease in the transformation temperatures or alter themagnitude of the temperature hysteresis. By selecting the appropriateconcentration, type, and/or combination of rare earth alloying elements,A_(f) and the other transformation temperatures can be fine-tuned towithin the desired temperature range. Furthermore, one or moreadditional alloying elements can be included in combination with the oneor more rare earth alloying elements to obtain the desiredtransformation temperatures. For example, additions of chromium,palladium, cobalt and/or iron may be effective in reducing A_(f).Additions of vanadium and/or cobalt may be effective in reducing M_(s).Copper is useful for eliminating the R-phase.

In practice, differential scanning calorimetry (DSC) techniques known inthe art may be used to determine the phase transformation temperaturesof the phases present in the nickel-titanium alloys. DSC measurementsmay be carried out according to the American Society for Testing andMaterials (ASTM) standard F2004-05 entitled “Standard Test Method forTransformation Temperature of Nickel-Titanium Alloys by ThermalAnalysis,” which is hereby incorporated by reference. Alternatively,methods known as constant load dilatometry and bend and free recoverymay be employed to determine the transformation temperatures. Bend andfree recovery tests may be carried out in accordance with the ASTMstandard F2082-03 entitled “Standard Test Method for Determination ofTransformation Temperature of Nickel-Titanium Shape Memory Alloys byBend and Free Recovery,” which is hereby incorporated by reference.Electrical resistivity measurements are also known in the art fordetermining the phase transformation temperatures of metals and alloys.Such measurements may be carried out by heating and cooling the alloy ofinterest while recording voltage using a four-probe constant currenttechnique, for example. Using electrical resisitivity measurements, itis possible to characterize phase transformations occurring in thenickel-titanium alloy as a function of applied stress as well astemperature.

According to a preferred embodiment, the nickel-titanium alloy isbiocompatible. When introduced into a patient, a biocompatible materialor device will not cause an adverse reaction or response in a majorityof the patients. The biocompatibility of the nickel-titanium alloy maybe assessed according to American Society for Testing and Materials(ASTM) standards F748-04 entitled “Standard Practice for SelectingGeneric Biological Test Methods for Materials and Devices,” F813-01entitled “Standard Practice for Direct Contact Cell Culture Evaluationof Materials for Medical Devices,” and/or F895-84 entitled “StandardTest Method for Agar Diffusion Cell Culture Screening for Cytotoxicity.”Additionally, the International Standards Organization (ISO) StandardNo. 10993 and/or the U.S. Pharmacopeia (USP) 23 and/or the U.S. Food andDrug Administration (FDA) blue book memorandum No. G95-1, entitled “Useof International Standard ISO-10993, Biological Evaluation of MedicalDevices Part-1: Evaluation and Testing” may be useful in evaluating thebiocompatibility of the nickel-titanium alloy and/or a medical devicecomprising the alloy. The aforementioned standards set forth practicesand methods designed for evaluating cytotoxicity, infectivity,pyrogenicity, irritation potential, reactivity, hemolytic activity,carcinogenicity and/or immunogenicity, and are hereby incorporated byreference. Since biocompatibility is a function of the type of bodilytissue contact and the duration of contact, the amount of testingrequired generally depends on the application. For example, thebiocompatibility testing requirements for a short term contacting basketare substantially different from those of a permanently implanted stent.

To produce the nickel-titanium alloys of the present disclosure andmedical devices comprising the alloys, a melt including the desiredamounts of alloying elements is formed and then cooled into a solid(e.g., an ingot). For example, from about 34 at. % to about 60 at. %nickel, from about 34 at. % to about 60 at. % titanium, and from about0.1 at. % to about 15 at. % at least one rare earth element may be addedto the melt. Up to about 14.9 at. % additional alloy elements may alsobe included in the melt. High purity raw materials (e.g., Ti>99.7 wt. %purity and Ni>99.99 wt. % purity) are preferably melted in an inert gasor vacuum atmosphere.

Melting methods known in the art, including but not limited to vacuuminduction melting (VIM), vacuum consumable arc melting (VAR), andelectron beam melting, may be employed to form the melt. Remelting isgenerally desirable to obtain satisfactory microstructural homogeneityin the ingot. For example, successive VAR processes or a VIM/VAR doublemelting process may be employed.

The ingot may then be hot worked into a first shape (e.g., bar, rod,tube hollow, or plate) by, for example, extruding, hot rolling, orforging. Hot working is generally employed to refine the cast structureof the ingot and to improve mechanical properties. The hot working isgenerally carried out at temperatures in the range of from about 700° C.to about 950° C., and may require multiple hot working and reheatingcycles. The reheating may be carried out over an eight hour period, forexample. Preferably, the ingot undergoes a minimum deformation of about90% during hot working in order to homogenize the as-cast, dendriticmicrostructure. Prior to hot working, it may be beneficial to carry outa solution heat treatment that involves soaking the ingot at an elevatedtemperature for a given time duration, followed by quenching. Thesolution heat treatment may aid in homogenizing the microstructure ofthe alloy and may be carried out at a temperature in the range of fromabout 850° C. to about 1150° C., for example. Preferably, the solutionheat treatment is carried out at a temperature in the range of fromabout 1000° C. to about 1150° C.

The first shape (e.g., bar, rod, tube, or plate) may then be cold workedinto a component by cold drawing or cold rolling, for example. The coldworking typically involves several passes in combination with interpassannealing treatments at temperatures in the range of from about 600° C.to about 800° C. The interpass annealing treatments soften the materialthrough recrystallization and growth of the austenite grains betweencold work passes, where 30-40% deformation is typically imparted. Ifcold drawing is employed to form a wire, for example, a polycrystallinediamond die with a molybdenum disulphide or other suitable lubricant maybe employed in order to reduce the drawing stress.

Machining operations, such as, for example, drilling, cylindricalcenterless grinding, or laser cutting may also be employed to fabricatethe component. Other operations, such as wire braiding or winding, mayalso be carried out.

A heat treatment is employed to impart a “memory” of a desired finalshape and to optimize the shape memory/superelastic and mechanicalproperties of the component. The number, duration and the temperature ofthe heat treatments may alter the transformation temperatures.Typically, heat treatment temperatures of 350° C. to 550° C. areappropriate to set the final shape and optimize the shapememory/superelastic and mechanical properties. Preferably, the heattreating involves annealing the component while constrained in a finalshape at a temperature in the range of from about 350° C. to about 550°C. More preferably, heat treatment or annealing temperatures in therange of from 450° C. to 550° C. are appropriate. In alloys having anexcess of nickel atoms (e.g., from about 50.6 to about 50.8 at. % Ni),for example, the heat treatments described above may cause nickel-richprecipitates to form, thereby reducing the nickel content of the matrixand causing the transformation temperatures to increase. Theprecipitates may also improve the tensile strength of thenickel-titanium alloy. Precipitation of these nickel-rich particles maybe desirable so as to obtain a thermoelastic martensitic phasetransformation from austenite.

According to a preferred embodiment, the nickel-titanium alloys of thepresent disclosure have an ultimate tensile strength of at least about1350 MPa. As is generally known to those of skill in the art, theultimate tensile strength (or tensile strength) of a materialcorresponds to the maximum engineering stress that can be sustained bythe material in tension without fracture. Engineering stress is definedas

$\frac{F}{A_{0}},$

where F represents tensile force and A₀ represents the originalcross-sectional area of the specimen prior to application of the force.Tensile testing of the alloys is preferably carried out in accordancewith American Society of Testing and Materials (ASTM) standards F2063,“Standard Specification for Wrought Nickel-Titanium Shape Memory Alloysfor Medical Devices and Surgical Implants” and/or F2516 “Standard TestMethod for Tension Testing of Nickel-Titanium Superelastic Materials,”which are hereby incorporated by reference.

In the case of nickel-titanium alloys in which a two-way shape memoryeffect is desired, additional “training” at lower temperatures may becarried out to set a second shape.

EXAMPLE 1

Ingots of several rare-earth doped nickel-titanium alloys were producedusing vacuum induction melting (VIM). Specifically, Ni—Ti—Er, Ni—Ti—La,Ni—Ti—Gd, and Ni—Ti—Nd, each containing 7.5 at. % rare earth element,were melted. A Ni—Ti-7.5 at. % Pt ingot and a binary nickel-titaniumalloy were also produced by VIM for comparison. The ingots of 2.25inches in diameter and 3 inches in height were rolled to form plates.Each of the Ni—Ti—X plates showed some interdendritic cracking as aconsequence of rolling, although the Er-doped nickel-titanium alloyseemed to withstand rolling the best. The rolled plates were soaked for24 hours at 850° C. and then hot worked to a size of slightly greaterthan 1 inch (2.54 cm) in height. The composition of each specimen isgiven in weight percent in Table 5. The concentration of carbon, oxygen,and nitrogen impurities is also shown in parts per million (ppm).

TABLE 5 Composition Data for Ni—Ti and Ni—Ti—X (X = Er, La, Gd, Nd, orPt) Specimens Nitro- RE or Carbon Oxygen gen Ti Pt Sample (ppm) (ppm)(ppm) Ni (wt. %) (wt. %) Ni—Ti—Er 1320 236 60 Balance 32.04 20.12Ni—Ti—La 760 307 8 Balance 33.17 17.30 Ni—Ti—La 33 2130 23 Balance 33.1717.30 Ni—Ti—Gd 380 149 6 Balance 32.43 19.15 Ni—Ti—Nd 140 124 4 Balance32.95 17.85 Ni—Ti—Pt 720 270 12 Balance 31.00 22.71 Ni—Ti 980 254 15Balance Wash — chemistry

Prior to rolling, the surfaces of the as-cast specimens were polished toprepare the samples for conventional Brinell hardness tests. Such testsinvolve pressing a spherical indenter of a specified diameter under aknown load into the surface of the specimen, and measuring the diameter(d) of the indentation after the test. A Brinell hardness number (BHN)may then be obtained by dividing the load used, in kilograms, by theactual surface area of the indentation, in square millimeters. Brinellhardness numbers obtained from hardness tests on polished, as-castspecimens are presented in Table 6 below. A steel ball of 1.68 mm indiameter was pushed into the surface of each specimen with a 30 kg forcefor a dwell time of 10 seconds. Four indentations were made for eachsample, with two measurements of diameter (d₁, d₂) for each indentation.Higher average BHN numbers are obtained from specimens exhibitinggreater resistance to plastic deformation (i.e., showing increasedhardness), and lower average BHN numbers are obtained from softerspecimens. As indicated in Table 6, the Ni—Ti-RE specimens exhibitedlower hardnesses than did the binary Ni—Ti specimen. The Ni—Ti—Pt sampleexhibited a higher hardness than did the binary Ni—Ti specimen.

TABLE 6 Brinell Hardness Data for As-Cast, Polished Specimens AverageAlloy d₁ d₂ BHN BHN Ni—Ti—Gd 0.4096 0.4274 317.4 304 0.437 0.4373 2910.4266 0.4467 291 0.4153 0.4244 315 Ni—Ti—Nd 0.4816 0.5093 226.5 2470.4531 0.4579 268.1 0.4614 0.4676 257.8 0.4919 0.4858 235.7 Ni—Ti—Pt0.3679 0.3522 429 460 0.3735 0.3618 411.4 0.3447 0.3023 531.5 0.35350.3349 469.5 Ni—Ti—Er 0.4266 0.4289 303.9 294 0.4254 0.4207 311.5 0.45820.4466 271.8 0.4355 0.4405 289.9 Ni—Ti 0.4395 0.4386 288.5 311 0.42650.4315 302.2 0.4188 0.3953 335.6 0.4159 0.4203 318

The microstructures of the hot worked specimens were investigated usinga scanning electron microscope (SEM) equipped with an energy dispersivex-ray spectrometer (EDS). The SEM allowed regions of the alloys to beviewed at high magnifications and the EDS provided localized chemicalinformation. Used together, the tools showed that the rare earthelements tended to segregate to the grain boundaries of the Ni—Ti-REspecimens. The alloy microstructure showed a dendritic form and includedoxide and carbide precipitates. It is believed that compositionalnonuniformity may inhibit shape memory phase transformations near humanbody temperature. Indeed, DSC experiments conducted by heating andcooling the specimens over temperatures ranging from −150° C. to 80° C.revealed no phase transformations. Accordingly, the inventors believethat a homogenization heat treatment at a temperature in excess of 850°C. (e.g., 1000° C. to 1150° C.) and for a longer time duration (e.g.,2-3 days) may be advantageous for improving the compositionalhomogeneity of the Ni—Ti-RE ingots and obtaining a suitable phasestructure for shape memory behavior around body temperature.

Experiments to compare the x-ray contrast of two of the Ni—Ti-RE alloysand Ni—Ti—Pt with the x-ray contrast of a binary Nitinol alloy wereconducted using a Picker Clinix RF fluoroscope and a phantom developedby the Center for Devices and Radiological Heath (CDRH) of the U.S. Foodand Drug Administration (FDA). The phantom was used to simulate x-rayattenuation through the lower abdomen of a typical adult. In particular,the phantom was designed to represent the upper gastrointestinal tractof a 5′8″ adult weighing about 165 lbs with a posterior-anteriorthickness of 23 cm. The dimensions of the phantom, which is composedprimarily of polymethyl methacrylate (PMMA) and aluminum, are given inFIG. 20.

The three ternary nickel-titanium alloy specimens used in theradiopacity experiments included, respectively, 7.5 at. % Er, 7.5 at. %Gd, and 7.5 at. % Pt. The experiments were carried out using the CDRHphantom in fluoroscopic mode and static mode. The intensity of theradiation transmitted through each specimen and the background intensitywere measured at various tube voltages. Values of x-ray contrast wereobtained by subtracting the radiation transmitted through the specimenfrom the background intensity at each voltage. The x-ray contrast valueswere then normalized by the x-ray contrast obtained for the binary Ni—Tisample to obtain relative x-ray contrast values for each specimen, asshown in Tables 7 and 8.

As indicated by the x-ray contrast data, each ternary alloy showed animprovement in radiopacity relative to the binary Nitinol alloy. Table 7shows the relative x-ray contrast values of the alloys as determinedusing the CDRH phantom at various voltages in fluoroscopic mode, andFIG. 21 shows the average value of relative x-ray contrast for eachalloy over the range of voltages used. Overall, the Ni—Ti—Gd alloyexhibited the highest x-ray contrast, with an average relative x-raycontrast of 1.50 for the voltage range of 40-110 kV. The Ni—Ti—Er alloyshowed an average relative x-ray contrast of 1.48 for the same voltagerange, while the Ni—Ti—Pt alloy exhibited an average relative x-raycontrast of 1.45.

TABLE 7 Values of Relative X-Ray Contrast (Fluoroscopic Mode) Specimen40 kV 50 kV 60 kV 70 kV 80 kV 90 kV 100 kV 110 kV Avg. Ni—Ti x-ray 293301 372 370 300 370 295 333 329.3 contrast relative 1.00 1.00 1.00 1.001.00 1.00 1.00 1.00 1.00 value Ni—Ti—Gd x-ray 532 464 477 495 492 484490 525 494.9 contrast relative 1.82 1.54 1.28 1.34 1.64 1.31 1.66 1.581.50 value Ni—Ti—Er x-ray 546 492 488 488 377 523 500 490 488.0 contrastrelative 1.86 1.63 1.31 1.32 1.26 1.41 1.69 1.47 1.48 value Ni—Ti—Ptx-ray 482 435 480 560 499 453 490 422 477.6 contrast relative 1.64 1.451.29 1.51 1.66 1.22 1.66 1.27 1.45 value

Table 8 shows the relative x-ray contrast values of the alloys asdetermined using the CDRH phantom at several voltages in static mode,and FIG. 22 shows the average value of relative x-ray contrast for eachalloy over the range of voltages used. Overall, the Ni—Ti—Pt alloyexhibited the highest x-ray contrast under these conditions, with anaverage relative x-ray contrast of 1.35 for the voltage range of 60-100kV. The Ni—Ti—Er alloy showed an average relative x-ray contrast of 1.34for the same voltage range, while the Ni—Ti—Gd alloy exhibited anaverage x-ray contrast of 1.29.

TABLE 8 Values of Relative X-Ray Contrast (Static Mode) Specimen 60 kV70 kV 80 kV 90 kV 100 kV Avg. Ni—Ti x-ray 540 490 437 399 380 449.2contrast relative value 1.00 1.00 1.00 1.00 1.00 1.00 Ni—Ti—Gd x-ray 605610 572 572 540 579.8 contrast relative value 1.12 1.24 1.31 1.43 1.421.29 Ni—Ti—Er x-ray 555 620 600 651 583 601.8 contrast relative value1.03 1.27 1.37 1.63 1.53 1.34 Ni—Ti—Pt x-ray 644 662 570 580 571 605.4contrast relative value 1.19 1.35 1.30 1.45 1.50 1.35

Preferably, the x-ray contrast of a Ni—Ti-RE alloy is in the range offrom greater than 1 to about 2 times that of a near-equiatomic binarynickel-titanium alloy when the alloys are exposed to radiation having anenergy in the range of from 40 keV to 110 keV. More preferably, thex-ray contrast of the Ni—Ti-RE alloy is in the range of from about 1.2to about 1.9 times that of the near-equiatomic binary nickel-titaniumalloy when the alloys are exposed to radiation having an energy in therange of from 40 keV to 110 keV.

EXAMPLE 2

Twelve additional alloys having the compositions given in Table 9 beloware being melted. After melting and casting, the alloys may undergo ahomogenization heat treatment at 1000° C. for 72 hours. The homogenizedalloys may then be mechanically worked into specimens as describedabove.

TABLE 9 Composition Data for Ni—Ti—Er and Ni—Ti—Er—X (X = Pd or Cr)Specimens Ni Ti Er Pd Cr (at. %) (at. %) (at. %) (at. %) (at. %) SeriesA 51 45 4 51 44 4 1 51 44 4 1 Series B 52.5 43.5 4 52.5 42.5 4 1 52.542.5 4 1 Series C 55 41 4 55 40 4 1 55 40 4 1 Series X 47 49 4 45 51 443 53 4

A nickel-titanium alloy comprising nickel, titanium, and at least onerare earth element (RE) has been described. A medical device comprisingat least one component including the nickel-titanium alloy has also beendescribed. The radiopaque Ni—Ti-RE alloy has improved radiopacitycompared to previous nickel-titanium alloys. Consequently, the medicaldevice has better visibility during non-invasive imaging procedures,such as x-ray fluoroscopy. The nickel-titanium alloy preferably furtherhas superelastic or shape memory properties that are advantageous forthe medical device.

According to one embodiment, the nickel-titanium alloy comprises fromabout 39 at. % to about 55 at. % nickel; from about 39 at. % to about 55at. % titanium; and from about 5 at. % to about 10 at. % at least onerare earth element preferably selected from the group consisting of La,Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The alloyfurther comprises one or more additional alloying elements at aconcentration of up to about 9.9 at. %. Preferably, the one or moreadditional alloying elements are selected from the group consisting ofAl, Cr, Mn, Fe, Co, Cu, Zn, Ga, Ge, Tc, Cd, In, Sn, Sb, Hg, Tl, Pb, Bi,Po, V, Ir, Pt, Au, Re, W, Pd, Rh, Ta, Ag, Ru, Hf, Os, Zr, Nb, and Mo.The additional alloying element preferably has a lower concentration inthe alloy than the rare earth element when the one or more additionalalloying elements are selected from the group consisting of Ir, Pt, Au,Re, W, Pd, Rh, Ta, Ag, Ru, Hf, Os, Zr, Nb, and Mo. It is also preferredthat the radiopacity of the nickel-titanium alloy is from about 1.2 toabout 5 times greater than that of the near-equiatomic binarynickel-titanium alloy when exposed to radiation having an energy in therange of from 15 keV to 150 keV. The radiation may have a peak intensityat an energy in the range of from 30 keV to 60 keV. The alloy ispreferably biocompatible and superelastic with an austenite finishtemperature in the range of from about −15° C. to about 37° C. It isalso advantageous for the nickel-titanium alloy to have an ultimatetensile strength of at least about 1350 MPa.

According to another embodiment, the nickel-titanium alloy comprisesfrom about 39 at. % to about 55 at. % nickel; from about 39 at. % toabout 55 at. % titanium; and from about 2.5 at. % to about 7.5 at. % atleast one rare earth element preferably selected from the groupconsisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, andLu. Preferably, the alloy further comprises one or more additionalalloying elements at a concentration of up to about 9.9 at. %, whereinthe one or more additional alloying elements are selected from the groupconsisting of Al, Cr, Mn, Fe, Co, Cu, Zn, Ga, Ge, Tc, Cd, In, Sn, Sb,Hg, Tl, Pb, Bi, Po, V, Ir, Pt, Au, Re, W, Pd, Rh, Ta, Ag, Ru, Hf, Os,Zr, Nb, and Mo. The additional alloying element preferably has a lowerconcentration in the alloy than the rare earth element when the one ormore additional alloying elements are selected from the group consistingof Ir, Pt, Au, Re, W, Pd, Rh, Ta, Ag, Ru, Hf, Os, Zr, Nb, and Mo. It isalso preferred that the radiopacity of the nickel-titanium alloy is fromabout 1.2 to about 5 times greater than that of the near-equiatomicbinary nickel-titanium alloy when exposed to radiation having an energyin the range of from 15 keV to 150 keV. The radiation may have a peakintensity at an energy in the range of from 30 keV to 60 keV. The alloyis preferably biocompatible and superelastic with an austenite finishtemperature in the range of from about −15° C. to about 37° C. It isalso advantageous for the nickel-titanium alloy to have an ultimatetensile strength of at least about 1350 MPa.

According to another embodiment, the nickel-titanium alloy has aradiopacity and comprises from about 39 at. % to about 55 at. % nickel;from about 39 at. % to about 55 at. % titanium; and from about 5 at. %to about 10 at. % at least one rare earth element, whereby thenickel-titanium alloy has a radiopacity greater than that of anear-equiatomic binary nickel-titanium alloy. Preferably, the at leastone rare earth element is selected from the group consisting of La, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. It is also preferredthat the alloy further comprise one or more additional alloying elementsat a concentration of up to about 9.9 at. %. Preferably, the one or moreadditional alloying elements are selected from the group consisting ofAl, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Tc, Cd, In, Sn, Sb, Hg, Tl, Pb,Bi, Po, V, Ir, Pt, Au, Re, W, Pd, Rh, Ta, Ag, Ru, Hf, Os, Zr, Nb, andMo. The one or more additional alloying elements preferably have a lowerconcentration in the alloy than the rare earth element when the one ormore additional alloying elements are selected from the group consistingof Ir, Pt, Au, Re, W, Pd, Rh, Ta, Ag, Ru, Hf, Os, Zr, Nb, and Mo. It isadvantageous for the nickel-titanium alloy to have a radiopacity in therange of from greater than about 1.2 to about 5 times that of thenear-equiatomic binary nickel-titanium alloy when exposed to radiationhaving an energy in the range of from 15 keV to 150 keV. Preferably, theradiation has a peak intensity at an energy in the range of from 30 keVto 60 keV. It is also preferred that the nickel-titanium alloy has alower temperature phase and a higher temperature phase, wherein strainintroduced in the nickel-titanium alloy in the lower temperature phaseis recovered upon a phase transformation to the higher temperaturephase. The lower temperature phase is preferably martensite and thehigher temperature phase is preferably austenite. It is also preferredthat the alloy has an austenite finish temperature in the range of fromabout −15° C. to about 37° C., and the alloy may further include anintermediate temperature R-phase. Preferably, the nickel-titanium alloyhas an ultimate tensile strength of at least about 1350 MPa and isbiocompatible.

According to another embodiment, the nickel-titanium alloy has aradiopacity and comprises from about 39 at. % to about 55 at. % nickel;from about 39 at. % to about 55 at. % titanium; and from about 2.5 at. %to about 7.5 at. % at least one rare earth element, whereby thenickel-titanium alloy has a radiopacity greater than that of anear-equiatomic binary nickel-titanium alloy. Preferably, the at leastone rare earth element is selected from the group consisting of La, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. It is also preferredthat the alloy further comprise one or more additional alloying elementsat a concentration of up to about 9.9 at. %. Preferably, the one or moreadditional alloying elements are selected from the group consisting ofAl, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Tc, Cd, In, Sn, Sb, Hg, Tl, Pb,Bi, Po, V, Ir, Pt, Au, Re, W, Pd, Rh, Ta, Ag, Ru, Hf, Os, Zr, Nb, andMo. The one or more additional alloying elements preferably have a lowerconcentration in the alloy than the rare earth element when the one ormore additional alloying elements are selected from the group consistingof Ir, Pt, Au, Re, W, Pd, Rh, Ta, Ag, Ru, Hf, Os, Zr, Nb, and Mo. It isadvantageous for the radiopacity of the nickel-titanium alloy to in therange of from greater than about 1.2 to about 5 times that of thenear-equiatomic binary nickel-titanium alloy when exposed to radiationhaving an energy in the range of from 15 keV to 150 keV. Preferably, theradiation has a peak intensity at an energy in the range of from 30 keVto 60 keV. It is also preferred that the nickel-titanium alloy has alower temperature phase and a higher temperature phase, wherein strainintroduced in the nickel-titanium alloy in the lower temperature phaseis recovered upon a phase transformation to the higher temperaturephase. The lower temperature phase is preferably martensite and thehigher temperature phase is preferably austenite. It is also preferredthat the alloy has an austenite finish temperature in the range of fromabout −15° C. to about 37° C., and the alloy may further include anintermediate temperature R-phase. Preferably, the nickel-titanium alloyhas an ultimate tensile strength of at least about 1350 MPa and isbiocompatible.

According to another embodiment, the nickel-titanium alloy includesnickel at a concentration of from about 34 at. % to about 60 at. %,titanium at a concentration of from about 34 at. % to about 60 at. %,and at least one rare earth element at a concentration of from about 2.5at. % to about 7.5 at. %. Preferably, the at least one rare earthelement is selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, and U. The nickel-titaniumalloy has a radiopacity greater than that of a near-equiatomic binarynickel-titanium alloy and further includes at least one additionalalloying element at a concentration of no more than about 4.9 at. %,Preferably, the additional alloying element being selected from thegroup consisting of Cr, Co, Fe, and Pd. It is also advantageous for thealloy to be superelastic with an austenite finish temperature at orbelow about 37° C. Preferably, the nickel-titanium alloy includes arecoverable strain of at least about 0.5% upon removal of a deformingstress at or below body temperature.

According to another embodiment, the nickel-titanium alloy includes fromabout 50 at. % to about 56 at. % nickel; from about 40 at. % to about 46at. % titanium; from about 0.1 at. % to about 4 at. % Er; and up toabout 1 at. % at least one transition metal, wherein the at least onetransition metal is selected from the group consisting of Cr, Fe, Co,and Pd. Preferably, the nickel-titanium alloy has a radiopacity of fromabout 1.2 to about 5 times greater than that of a near-equiatomic binarynickel-titanium alloy when exposed to radiation having an energy in therange of from 15 keV to 150 keV. The radiation may have a peak intensityat an energy in the range of from 30 keV to 60 keV. It is preferablethat the nickel-titanium alloy is superelastic with an austenite finishtemperature in the range of from about −15° C. to about 37° C. Thenickel-titanium alloy also preferably has an ultimate tensile strengthof at least about 1350 MPa and is biocompatible.

According to another embodiment, the nickel-titanium alloy includes fromabout 50 at. % to about 56 at. % nickel; from about 40 at. % to about 46at. % titanium; from about 0.1 at. % to about 4 at. % Er; and up toabout 1 at. % at least one transition metal, wherein the at least onetransition metal is selected from the group consisting of Cr, Pd, Co,and Fe. Preferably, the nickel-titanium alloy has a radiopacity of about1.2 to about 5 times greater than that of a near-equiatomic binarynickel-titanium alloy when exposed to radiation having an energy in therange of from 15 keV to 150 keV. The radiation preferably has a peakintensity at an energy in the range of from 30 keV to 60 keV. It is alsopreferred that the nickel-titanium alloy has a lower temperature phaseand a higher temperature phase, wherein strain introduced in thenickel-titanium alloy in the lower temperature phase is recovered upon aphase transformation to the higher temperature phase. The lowertemperature phase may be martensite, and the higher temperature phasemay be austenite. The alloy may further comprise an intermediatetemperature R-phase. Preferably, the nickel-titanium alloy has anaustenite finish temperature in the range of from about −15° C. to about37° C. and an ultimate tensile strength of at least about 1350 MPa. Itis also preferred that the nickel-titanium alloy is biocompatible.

According to one embodiment, the medical device includes at least onecomponent comprising a nickel-titanium alloy including from about 34 at.% to about 60 at. % nickel, from about 34 at. % to about 60 at. %titanium, and from about 0.1 at. % to about 15 at. % at least one rareearth element, wherein the at least one rare earth element is selectedfrom the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Ac, Th, Pa and U. Preferably, the nickel-titanium alloy hasa radiopacity greater than that of a near-equiatomic binarynickel-titanium alloy. It is also preferred that the alloy issuperelastic, and that the component is a self-expandable stent.

According to another embodiment, the medical device includes at leastone component comprising a nickel-titanium alloy including from about 34at. % to about 60 at. % nickel; from about 34 at. % to about 60 at. %titanium; from about 0.1 at. % to about 10 at. % at least one rare earthelement; and at least one transition metal at a concentration of no morethan about 4.9 at. %, wherein the at least one rare earth element isselected from the group consisting of Ce, La, Pr, Nd, Pm, Sm, Eu, Gd,Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, and U. Preferably, thenickel-titanium alloy has a radiopacity greater than that of anear-equiatomic binary nickel-titanium alloy and is superelastic. Thecomponent may be a self-expandable stent.

According to another embodiment, the medical device includes at leastone component comprising a nickel-titanium alloy including nickel at aconcentration of from about 34 at. % to about 60 at. %, titanium at aconcentration of from about 34 at. % to about 60 at. %, and at least onerare earth element at a concentration of from about 0.1 at. % to about15 at. %, wherein the at least one rare earth element is selected fromthe group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Yb, Lu, Ac, Th, Pa and U. Preferably, the component includes at leastone of a wire and a cannula. It is also preferred that the concentrationof the at least one rare earth element is from about 2.5 at. % to about7.5 at. %. The nickel-titanium alloy preferably has a radiopacitygreater than that of a near-equiatomic binary nickel-titanium alloy andis superelastic with an austenite finish temperature at or below about37° C.

According to another embodiment, the medical device is radiopaque andincludes at least one component comprising a nickel-titanium alloyincluding nickel at a concentration of from about 34 at. % to about 60at. %, titanium at a concentration of from about 34 at. % to about 60at. %, and at least one rare earth element at a concentration of fromabout 0.1 at. % to about 15 at. %, wherein the at least one rare earthelement is selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa and U. The nickel-titaniumalloy comprises a radiopacity greater than that of a near-equiatomicbinary nickel-titanium alloy, and the nickel-titanium alloy furthercomprises a recoverable strain of at least about 0.5% upon removal of adeforming stress at or below body temperature. Preferably, therecoverable strain is in the range of from about 2% to about 10%. It isalso preferred that the nickel-titanium alloy is superelastic and has anaustenite finish temperature at or below about 37° C.

A method of using a medical device has also been described herein.According to one aspect, the method includes providing a medical devicehaving at least one component comprising a nickel-titanium alloyincluding from about 34 at. % to about 60 at. % nickel, from about 34at. % to about 60 at. % titanium, and from about 0.1 at. % to about 15at. % at least one rare earth element. Preferably the at least one rareearth element is selected from the group consisting of La, Pr, Nd, Pm,Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa and U. The medicaldevice is then delivered to a treatment site within the patient. Themethod may further include loading the medical device into a deliverysystem and inserting the medical device into the patient after loadingthe medical device into a delivery system. Preferably, thenickel-titanium alloy has a radiopacity greater than that of anear-equiatomic binary nickel-titanium alloy. It is also preferred thatthe nickel-titanium alloy is superelastic. The component may be a stent,a retrieval device, and an embolic protection filter.

According to another aspect, the method of using the medical device mayinclude providing a medical device including at least one componentcomprising a nickel-titanium alloy having from about 34 at. % to about60 at. % nickel, from about 34 at. % to about 60 at. % titanium, fromabout 0.1 at. % to about 10 at. % at least one rare earth element, andat least one transition metal at a concentration of no more than about4.9 at. %, wherein the at least one rare earth element is selected fromthe group consisting of Ce, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Ac, Th, Pa, and U. The medical device is delivered to atreatment site within the patient. The method may further includeloading the medical device into a delivery system and inserting themedical device into the patient. Preferably, the nickel-titanium alloyhas a radiopacity greater than that of a near-equiatomic binarynickel-titanium alloy. It is also preferred that the nickel-titaniumalloy is superelastic. The component may be a stent, a retrieval device,and an embolic protection filter.

A method of imaging a medical device within a patient also has beendescribed herein. According to one aspect, the method includesdelivering a medical device having at least one component comprising anickel-titanium alloy including from about 34 at. % to about 60 at. %nickel, from about 34 at. % to about 60 at. % titanium, and from about0.1 at. % to about 15 at. % at least one rare earth element to atreatment site within a patient. The at least one rare earth element isselected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa and U. The patient is exposed toradiation having an energy in the range of from 15 keV to 150 keV,thereby imaging the medical device. Preferably, the radiation has a peakintensity at an energy in the range of from 30 keV to 60 keV. Exposingthe patient to radiation preferably includes providing an x-ray sourcein opposition to the patient and at least one filter between the x-raysource and the patient, wherein the x-ray source comprises a tubevoltage in the range of from 60 kVp to 150 kVp. Preferably, the filteris selected from the group consisting of a 2.5 mm aluminum filter, 0.1mm copper filter, 0.2 mm copper filter, and 0.3 mm copper filter.

According to another aspect, the method of imaging the medical deviceincludes delivering a medical device having at least one componentcomprising a nickel-titanium alloy including from about 34 at. % toabout 60 at. % nickel, from about 34 at. % to about 60 at. % titanium,from about 0.1 at. % to about 10 at. % at least one rare earth element;and at least one transition metal at a concentration of no more thanabout 4.9 at. %, wherein the at least one rare earth element is selectedfrom the group consisting of Ce, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, Lu, Ac, Th, Pa, and U to a treatment site within a patient.The patient is exposed to radiation having an energy in the range offrom 15 keV to 150 keV, thereby imaging the medical device. Preferably,the radiation has a peak intensity at an energy in the range of from 30keV to 60 keV. Exposing the patient to radiation preferably includesproviding an x-ray source in opposition to the patient and at least onefilter between the x-ray source and the patient, wherein the x-raysource comprises a tube voltage in the range of from 60 kVp to 150 kVp.Preferably, the filter is selected from the group consisting of a 2.5 mmaluminum filter, 0.1 mm copper filter, 0.2 mm copper filter, and 0.3 mmcopper filter.

A method of making a medical device has also been described herein.According to one aspect, the method comprises forming a melt comprisingfrom about 34 at. % to about 60 at. % nickel, from about 34 at. % toabout 60 at. % titanium, and from about 0.1 at. % to about 15 at. % atleast one rare earth element, wherein the at least one rare earthelement is selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa and U. The melt is cooled toform a solid, and the solid is formed into a component, thereby forminga medical device. Forming the solid into the component preferablyincludes hot working the solid into a first shape, and cold working thefirst shape into the component. Hot working the solid may include atleast one of extruding, hot rolling, and forging. Cold working the firstshape may include drawing or rolling. Forming the solid into a componentpreferably further includes annealing the component. The annealing mayentail constraining the component in a final shape and heating thecomponent at a temperature in the range of from about 350° C. to about550° C. It may be advantageous to solution heat treat the solid at atemperature of at least about 1,000° C.

According to another embodiment, the method of making the medical deviceincludes forming a melt comprising from about 34 at. % to about 60 at. %nickel, from about 34 at. % to about 60 at. % titanium, from about 0.1at. % to about 10 at. % at least one rare earth element, at least onetransition metal at a concentration of no more than about 4.9 at. %,wherein the at least one rare earth element is selected from the groupconsisting of Ce, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,Lu, Ac, Th, Pa, and U. The melt is cooled to form a solid, and the solidis formed into a component, thereby forming a medical device. Formingthe solid into the component preferably includes hot working the solidinto a first shape, and cold working the first shape into the component.Hot working the solid may include at least one of extruding, hotrolling, and forging. Preferably, the solid receives a solution heattreatment prior to hot working. Cold working the first shape may includedrawing or rolling. Forming the solid into a component preferablyfurther includes annealing the component. The annealing may entailconstraining the component in a final shape and heating the component ata temperature in the range of from about 350° C. to about 550° C. It maybe advantageous to solution heat treat the solid at a temperature of atleast about 1,000° C.

Although the present invention has been described in considerable detailwith reference to certain embodiments thereof, other embodiments arepossible without departing from the present invention. The spirit andscope of the appended claims should not be limited, therefore, to thedescription of the preferred embodiments contained herein. Allembodiments that come within the meaning of the claims, either literallyor by equivalence, are intended to be embraced therein.

Furthermore, the advantages described above are not necessarily the onlyadvantages of the invention, and it is not necessarily expected that allof the described advantages will be achieved with every embodiment ofthe invention.

1. A nickel-titanium alloy comprising: nickel at a concentration of fromabout 34 at. % to about 60 at. %; titanium at a concentration of fromabout 34 at. % to about 60 at. %; and at least one rare earth element ata concentration of from about 0.1 at. % to about 15 at. %, wherein theat least one rare earth element is selected from the group consisting ofLa, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, andU.
 2. The nickel-titanium alloy of claim 1 wherein the concentration ofthe rare earth element is from about 2.5 at. % to about 7.5 at. %. 3.The nickel-titanium alloy of claim 1, further comprising at least oneadditional alloying element at a concentration of up to about 14.9 at.%.
 4. The nickel-titanium alloy of claim 3, wherein the concentration ofthe additional alloying element is no more than about 4.9 at. % and theadditional alloying element is selected from the group consisting of Cr,Co, Fe, and Pd.
 5. The nickel-titanium alloy of claim 1 having aradiopacity greater than that of a near-equiatomic binarynickel-titanium alloy.
 6. The nickel-titanium alloy of claim 5, whereinthe radiopacity of the nickel-titanium alloy is in the range of fromgreater than about 1 to about 8 times that of the near-equiatomic binarynickel-titanium alloy when exposed to radiation having an energy in therange of from 15 keV to 150 keV.
 7. The nickel-titanium alloy of claim1, wherein the nickel-titanium alloy comprises an x-ray contrast in therange of from greater than 1 to about 2 times that of a near-equiatomicbinary nickel-titanium alloy when exposed to radiation having an energyin the range of from 40 keV to 110 keV.
 8. The nickel-titanium alloy ofclaim 1 being superelastic and having an austenite finish temperature ator below about 37° C.
 9. The nickel-titanium alloy of claim 1 comprisinga recoverable strain of at least about 0.5% upon removal of a deformingstress at or below body temperature.
 10. The nickel-titanium alloy ofclaim 1 having a radiopacity greater than that of a near-equiatomicbinary nickel-titanium alloy, the concentration of the rare earthelement being from about 2.5 at. % to about 7.5 at. %, and furthercomprising at least one additional alloying element at a concentrationof no more than about 4.9 at. %, the additional alloying element beingselected from the group consisting of Cr, Co, Fe, and Pd, wherein thenickel-titanium alloy is superelastic and has an austenite finishtemperature at or below about 37° C.
 11. A medical device comprising atleast one component comprising a nickel-titanium alloy including nickelat a concentration of from about 34 at. % to about 60 at. %, titanium ata concentration of from about 34 at. % to about 60 at. %, and at leastone rare earth element at a concentration of from about 0.1 at. % toabout 15 at. %, wherein the at least one rare earth element is selectedfrom the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Ac, Th, Pa and U.
 12. The medical device of claim 11,wherein the component includes at least one of a wire and a cannula. 13.The medical device of claim 11, wherein the concentration of the atleast one rare earth element is from about 2.5 at. % to about 7.5 at. %and the nickel-titanium alloy has a radiopacity greater than that of anear-equiatomic binary nickel-titanium alloy.
 14. The medical device ofclaim 11, wherein the nickel-titanium alloy is superelastic and has anaustenite finish temperature at or below about 37° C.
 15. A radiopaquemedical device comprising at least one component comprising anickel-titanium alloy including nickel at a concentration of from about34 at. % to about 60 at. %, titanium at a concentration of from about 34at. % to about 60 at. %, and at least one rare earth element at aconcentration of from about 0.1 at. % to about 15 at. %, wherein the atleast one rare earth element is selected from the group consisting ofLa, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa andU, whereby the nickel-titanium alloy comprises a radiopacity greaterthan that of a near-equiatomic binary nickel-titanium alloy, and wherebythe nickel-titanium alloy further comprises a recoverable strain of atleast about 0.5% upon removal of a deforming stress at or below bodytemperature.
 16. The radiopaque medical device of claim 15, wherein thenickel-titanium alloy is superelastic and has an austenite finishtemperature at or below about 37° C.
 17. The radiopaque medical deviceof claim 15, wherein the recoverable strain is in the range of fromabout 2% to about 10%.
 18. A method of making a medical devicecomprising at least one component, the method comprising: forming a meltcomprising from about 34 at. % to about 60 at. % nickel, from about 34at. % to about 60 at. % titanium, and from about 0.1 at. % to about 15at. % at least one rare earth element, wherein the at least one rareearth element is selected from the group consisting of La, Pr, Nd, Pm,Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa and U; cooling themelt to form a solid, forming the solid into a component, therebyforming a medical device comprising the component.
 19. The method ofclaim 18, wherein forming the solid into the component comprises: hotworking the solid into a first shape; cold working the first shape intothe component; and annealing the component, wherein annealing thecomponent comprises constraining the component in a final shape andheating the component at a temperature in the range of from about 350°C. to about 550° C.
 20. The method of claim 18, further comprisingsolution heat treating the solid at a temperature of at least about1,000° C.